STRUCTURE magazine - November 2020

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NOVEMBER 2020

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Cover Feature

Contents NOVEM BER 2020

Columns and Departments Editorial Keeping the Public Safe – How Far Must We Go? By Richard C. Boggs, P.E., SECB Construction Issues Warehouse Checklist for Success

By Kurt Voigt, P.E., and Ben Pitchford, P.E., and Stephnie Reddick

Structural Quality Quality Structural Welding and Fabrication

By William C. LaPlante

Structural Practices Transferring Loads in Existing Buildings

By Ciro Cuono, P.E.

Structural Systems Lightening the (Dead) Load on Floors

By Tim Liescheidt, P.E.

Community Resilience A New Challenge to the Practice of Structural Engineering

By Bruce R. Ellingwood, Ph.D., P.E., John W. van de Lindt, Ph.D.,

INNOVATIVE DESIGN USING COMPOSITE STEEL JOISTS

and Therese P. McAllister, Ph.D., P.E.

Outside the Box Utility-Scale Photovoltaic Power Plants

By Michael Martignetti, P.E.

The 121 Seaport Boulevard building incorporated a unique design dictated by site challenges, costs, and an open concept with minimal columns. At the core of this beautiful building are CJ-Series composite steel joists rather than a typical composite beam design. Cover photo courtesy of Bruce T. Martin Photography for Skanska USA.

By Sumanth Cheruku, P.E., and Matthew T.L. Browne, M.Eng, P.Eng

In Every Issue Advertiser Index NCSEA News SEI Update CASE in Point Resource Guide – Software Updates

Feature

SHORING THE UNSHORABLE

By Roya A. Abyaneh, P.E., and Edward S. Breeze, P.E.

A case study of rooftop cooling tower support structure repairs highlights common but challenging obstacles and solutions.

Temporarily shoring the tower’s load to accommodate reinforcing and/or replacement of supports can be challenging.

November 2020 Bonus Content

Additional Content Available Only at – STRUCTUREmag.org

Business Practices Market Trends – Succeeding Into 2022

By Kacey Clagett

Response to October 2020 STRUCTURE Structural Failures Article

By Jason B. Lloyd, Ph.D., P.E., Robert J. Connor, Ph.D., P.E., and Karl H. Frank, Ph.D., P.E.

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, the Publisher, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

NOVEMBER 2020

EDITORIAL Keeping the Public Safe – How Far Must We Go?

By Richard C. Boggs, P.E., SECB, LEED AP

W

e live in a society that tends to react to the latest threats found how to apply them effectively. At that time, I recall hearing a virtual in the most recent news cycle. A terrorist boards a plane in guarantee that the northeast would experience a significant seismic Paris in 2001 with an explosive in his shoe, and passengers must event in the subsequent twenty years, and the alarm was sounded to remove their shoes before boarding planes for decades. A sociopath be ready. Of course, thirty years later, this has still not happened for enters an elementary school in Connecticut in 2012 and shoots who- most of our region, but the majority of engineers in the northeast ever crosses his path. As a result, school design is forever changed to accept that the risk was real and remains so. incorporate security features that were not imagined decades earlier.

One of the first buildings I designed under the new 1989 code was Did we not know this could happen? a single-story school addition. The adjacent main wing of the existAs structural engineers, we are in the business of managing risk. The ing school was a four-story unreinforced concrete masonry bearing states that grant us licensure entrust us to make decisions that result wall structure built in the 1940s. There was no International Existing in a built environment that is safe Building Code (IEBC) at that time, for its occupants. But how is safety but the BOCA Code did contain a defined? Most engineers would chapter that addressed renovations of answer that safe design equals existing buildings. The renovations Should design professionals conformance with the building contemplated in this area were clearly code. But the building code is a not sufficient to mandate seismic be responsible for failure to anticipate minimum standard only, and is retrofit. With my knowledge of the conditions not previously experienced the result of a series of compropoor performance of unreinforced mises between various stakeholders masonry bearing wall structures in and, therefore, not prescribed whose objectives can be in conearthquakes, I presented what I felt by building codes? flict. This results in documents that was a sound argument to the projaspire to the public’s protection but ect manager (a co-worker in my A/E recognize economic realities that firm) to take steps to introduce duccan defy the certainty of that result. tility to the existing structure. When The risks addressed by the code can he asked whether the building code only reflect past experience. But mandated such a retrofit, I gave the what about future risks? How can those be determined and incorpo- honest response that it did not, and he elected not to bring the issue rated into building codes? Should design professionals be responsible to the School Building Committee for consideration. for failure to anticipate conditions not previously experienced and, The Code of Ethics in the Connecticut P.E. Regulations states, “The therefore, not prescribed by building codes? engineer… shall at all times recognize his or her primary obligation to One extreme example of this was the twin towers of the World Trade protect the safety, health, and welfare of the public in the performance Center, destroyed in 2001. The towers’ design actually did consider of his or her professional duties.” In that context, I look back on this the impact of an airplane, but not a fully loaded jet operating at full experience and others like it and wonder if this was an appropriate speed, bent on destruction. Following this disaster, much was written response. How would I feel if that old portion of the school collapsed about the performance of the structures and fire protection systems, in a moderate earthquake? How would I respond to the parents of resilience of egress paths, etc. Building codes were modified to apply children lost in the collapse if they asked why a structure known to the lessons learned. Similarly, the bombing of the Alfred P. Murrah perform poorly in an earthquake was permitted to be used as a school?

Federal Building in Oklahoma City in 1995 led to changes in codes and As structural engineers, do we demand the application of logic (and prescriptive design procedures for government-funded projects intended cost) beyond what the building code requires? I have always felt that to prevent progressive collapse. But progressive collapse was not an I had a sound legal argument by meeting the standard of care in this unknown phenomenon before the destruction of the Murrah Building. case, but is that really enough?

Should this have been a design consideration, even though it was not On the west coast, there have been efforts to be more proactive in mandated by code when the building was constructed? Hindsight is addressing pre-existing conditions that pose significant seismic hazalways 20/20, but the potential scenarios one can imagine are limitless. ards. Still, even there, such efforts face opposition due to costs and a I was a young structural engineer in the late 1980s when seismic loads perception that such actions may be discriminatory. These types of first became a design consideration in the northeastern US states. In discussions are lacking in the northeastern states. Unfortunately, this Connecticut, the 1987 Building Officials and Code Administrators debate will probably have to wait until an actual earthquake, (BOCA) Code, with the 1988 Supplement, was adopted in October and its resulting death and destruction, makes the case on of 1989, and henceforth earthquakes became a design consideration the nightly news for a more proactive response.■ for new buildings and renovations designed in my state. In preparaRichard C. Boggs is a Senior Project Manager at Fuss & O’Neill in tion for these changes, seismologists and structural dynamics experts Trumbull, CT, and Director on the NCSEA board. His 38-year career were dispatched to educate the structural engineers in the northeast includes a wide range of structural renovations and retrofits. about these provisions, why they were appropriate in our region, and STRUCTURE magazine

N O V E M B E R 2 02 0

construction ISSUES Warehouse Checklist for Success 5 Factors for Joist and Deck Specification

By Kurt Voigt, P.E., and Ben Pitchford, P.E., and Stephnie Reddick

T

he growth of e-commerce has bolstered warehouse and distribution center construction, with new project starts forecast to increase in 2021. Here are 5 areas where collaboration between the specifying engineer and the joist/deck

supplier improves construction efficiency, shortens project timelines, and reduces total project costs.

RTU Loads Often, the structural drawings for a warehouse will only generally indicate the presence of a rooftop unit (RTU) on the roof plan. Not specified is the unit’s distance to a column line or the end of a joist. This information is not often known at the time the Design Drawings are released for bidding or construction.

Whatever the reason, the lack of specificity about the location of mechanical equipment will trigger a request for information (RFI) cycle. The joists cannot be engineered and manufactured until the specification is made clear. To avoid this delay, the project engineer has options.

Option A: Specify General Location Suppose you know that the RTU will be located within the first 10 feet along the length of a particular joist. In that case, this is usually enough information for the joist manufacturer to proceed. Joist estimators and engineers know to strengthen the joist to support the RTU placement at any panel point along that 10-foot section of the joist. The additional cost for material is nominal compared with an exactly located unit within that zone, and far offset by the expediency of keeping the project moving.

Option B: Specify a KCS Joist KCS-Series joists are modified K-Series joists built to address constant shear and moment resulting from gravity loads along their spans. KCS

Figure 1. Expansion joints.

STRUCTURE magazine

joists are designed for compression in the top chord and tension in the bottom chord, and constant positive and negative shear in the web members, to account for varying load locations and potential stress reversals no matter where shear may occur within the joist. All web members, except the end diagonal webs of a KCS joist, are engineered to resist 100% of the published shear capacity applied in tension or compression. The result is a very strong joist engineered to support RTU and other mechanical loads at any panel point along the joist.

KCS-Series joists are ideal for warehouses with multiple loads by way of conveyors, catwalks, and suspended processing equipment. The cost of a joist will be higher due to the increased steel content. But, in addition to the avoidance of prolonged RFI’s and potential project delays, the specification of KCS-Series joists gives the building owner the flexibility to, at a later date, add or move loads along the joist span.

Expansion Joints For larger warehouses, expansion joints, sometimes called “slip joints,” allow for thermally induced horizontal shifting of a large section of roof joist girders, joists, and steel deck relative to another section. For expansion joints to perform properly, the specifying engineer must allow for adequate seat depths at the slide bearing locations. Doing so will permit the differential movement between the adjoining roof areas.

Allow for Joist Seat Depth As shown in Figure 1, if the joist seat depth is too shallow, the diagonal end web of the sliding joist girder will be impeded by the column cap plate, with the risk of structural damage during thermal movement. Given the proper joist girder seat depth, the joist engineer can address the full range of anticipated thermally induced horizontal movements at each expansion joint. The Steel Joist Institute’s (SJI) Standard Specification 100-2015, Table 5.4-3, can be used to determine the minimum seat depth required to accommodate the anticipated amount of slip. The minimum “RP” value to use in the table equations should equal the sum of anticipated forward and backward slip amounts, plus:

• ½ inch for K-Series joists, • 2 inches for all LH-Series, DLH10-17, and joist girders with a self-weight ≤ 50 plf, • 4 inches for all DLH18-25 and joist girders with a self-weight > 50 plf.

So, a joist girder weighing 60 plf, requiring +/- 1.75-inch slip, requires a minimum “RP” value of (1.75-inch x 2) + 4 inches = 7.5 inches.

Entering Table 5.4-3 with RP = 7.5 inches results in a minimum seat depth, D: The printed issue contains D = RP + 4 inches (for joist girders) an error in this calculation D = 7.5 inches + 4 inches and has been corrected in D = 11.5 inches (round up to 12 inches) all digital copies.

Figure 2. Wall thickness.

Figure 3. Account for jogs.

Tilt-Up Walls Tilt-up walls are frequently used on warehouse projects for efficient construction, but the expedience gained will be diminished when dimensional considerations related to the steel joists and roof decking are not clearly called out in the structural drawings.

Wall Thickness Concrete tilt-up walls are first poured as horizontal slabs to create panels. Each panel is typically fabricated with embedded steel plates to support and attach the steel joists and/or joist girders. The panels are poured on-site and tilted vertically to establish the warehouse walls.

The steel joists and joist girders are soon added to tie the structure together; but, for the joists to properly fit-up with the tilt walls, the joist engineer needs to know the wall thickness. Wall thickness dimensions are sometimes completely missing from the structural drawings. Other times, the dimensions are only partly indicated, such as when walls with varying thicknesses are not clearly called out or when panels that have reveals are not clearly detailed. But, as shown in Figure 2, when these dimensions are clearly detailed in the structural drawings, accounting for any variations, no related time is lost in the engineering of the joists.

Account for Jogs

Show Wall Location Typically, one face of the wall, inside or outside, will be located relative to the adjacent column centerline. This information needs to be called out in the structural drawings, as shown in Figure 4, regardless of which face of the panel is dimensioned. Dimensions to the inside face of the wall are the most critical for joist length. When the tilt panel’s inside

face is dimensioned from the adjacent column centerline, changes in panel thickness made by the precast contractor during their design will not affect the joist or joist girder lengths. This helps avoid length discrepancies that are costly both financially and to the erection schedule. It also helps avoid RFI delays when uncertainty exists related to panel thicknesses or wall face locations.

Note the Bearing Conditions The clear specification of the bearing conditions for the joist and joist girder connections to the tilt-up walls will avoid costly connection

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Tilt-up walls sometimes have “jogs,” at which point two panels lap or offset each other to create a jog in the face of the panels. The structural drawings frequently do not indicate the dimensions for these offsets, including the “gap.”

But when these dimensions are called out, as shown in Figure 3, the joist engineer loses no time determining the exact length of the joists. Manufacturing can be underway, and the joist detailer can include precise details to the erector for the proper fit-up of the joists on-site.

Figure 4. Wall location.

NOVEMBER 2020

Figure 5. Uplift forces.

Figure 6. Uplift zones.

issues. Connections include members bearing on a precast haunch on the wall face, in a wall pocket, on the face of the wall with an embedded plate shear connection, or on a steel embed angle on the face of the wall. Leaving these connections unknown will predictably delay the project schedule and increase the potential for coordination issues.

specifying engineer and erector, the sequences were reduced to 18.

On another project, multiple cranes operating simultaneously were aligned with joist and deck sequencing to opposite ends of the building, thus keeping two erection crews moving efficiently toward the middle. And, as happens on projects when the sequencing of a warehouse changes, it is relatively easy to adjust – provided the organized joist and deck sections do not change (e.g., SEQ 1 = column lines 1-8).

Too often, this is not how it goes, as many erectors can report.

Without a clear sequencing plan upfront, the joists and deck will arrive in bulk. The erector will lose time shaking out each truck, sorting and transporting the correct pieces to the appropriate locations of the building. Sequencing can also be disrupted by decisions made late, after the joist drawings are submitted for approval. A GC may change the sequencing to satisfy the precast company’s request to deliver the walls batched to match the tilt-up schedule. Or the erector may decide late that the joists, decking, and bridging for a high roof should be delivered first, having realized late that there was not enough lay-down room on-site for the entire joist order.

When these changes to earlier planned sequencing occur, joist and deck delivery time is disrupted. Pieces must be manually pulled, re-organized, and re-identified. The disruption also impacts other project stakeholders, including the owner, as re-sequencing elevates the potential for material supply errors, inefficient erection, delayed occupancy, and lost revenue.

Uplift Forces Show Net Uplift RFI delays can be avoided by clearly specifying the uplift per the factors shown in Figure 5: A) Indicate the wind uplift pressures required for the design of the steel joists and joist girders separately; B) Indicate whether wind uplift is net or gross; C) State whether the wind uplift values were determined using LRFD or ASD load combinations. Unless the structural drawings specifically state the joists are to be designed using LRFD or ASD, the joist manufacturer may use either. It is helpful for the structural drawings to include a note indicating the roof dead load used to calculate the net wind uplift, in case the manufacturer needs to convert from LRFD net pressure to ASD, or vice versa. Generally, the full design dead load is not used to determine the design net wind uplift, since that dead load includes allowances for equipment and other incidental loads that may not be present at all areas of the roof or for the life of the building. Net uplift pressures on warehouse projects are typically specified using service level (ASD) net uplift, but the joist engineer cannot assume this. An incorrect assumption can lead to overdesign, or even worse, an under-designed joist and joist girder system.

Show Uplift Zones If the uplift values are given for different “zones” (i.e., interior, perimeter, corner), then a diagram of the zones, including the dimensioned widths of each zone, must be provided on the structural drawings, as shown in Figure 6. Do not simply show an undefined “a” dimension, as it is a code level determination that should be made by the specifying engineer while specifying the uplift.

Project Sequencing Efficient joist and deck sequencing go hand-in-hand with efficient warehouse construction.

The authors recently participated in a large project initially specified to have 126 sequences. By collaborating early with the STRUCTURE magazine

Conclusion For many larger warehouse projects, the steel joists, joist girders, and decking comprise a significant percentage of the structural material.

The most frequent constraints to successful project delivery, related to joist and deck, are well known and documented in this article.

Early collaboration among the supplier, specifying engineer, erector, and GC to address these constraints will prevent confusion, project delays, unnecessary costs, and assure the building owner the earliest possible occupancy.■ Kurt Voigt (kurt.voigt@newmill.com) and Ben Pitchford (ben.pitchford@newmill.com) are Engineering Managers at New Millennium Building Systems. Stephnie Reddick is a scheduler for the company. (stephnie.reddick@newmill.com)

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structural QUALITY

Quality Structural Welding and Fabrication A Rigorous QMS, Human Performance Attributes, and Training

By William C. LaPlante

W

eld quality is the bedrock of structural welding execution as described in American Iron and Steel Institute (AISI) specifications, and American Welding Society (AWS) Codes D1.1, D1.2, and D1.6. The term “quality” represents “conformance to a specification” where the term “specification” refers to the requirements of a code, drawing, or contract. Weld quality is not inspected into a structure or weldment but is the result of production process improvements. Businesses engaged in seismic structural fabrication, American Society of Mechanical Engineer (ASME) Section III nuclear component fabrication, or ASME Section VIII pressure vessel fabrication, understand this philosophy. Engineering endeavors exist where weld failure is not an option without placing undue risk on human life, the environment, or property. Quality of welding workmanship is not a catchphrase. Substandard weld quality cost lives. Also, it represents waste as the result of liability lawsuits, warranty repair or replacement costs, and lost revenue due to expensive rework or repairs (Figure 1). Quality welding is a critical component in the fabrication of safe, dependable, and trustworthy structures. This article focuses on achieving quality structural welding and fabrication.

Quality Management System Quality does not come from inspections alone, but in conjunction with production process control improvements. A Quality Management System (QMS) focuses on the welding process as opposed to after-the-fact inspections. Employing process controls is the method by which weld quality is built into a structure. Monitoring and controlling process parameters, qualifying welding procedures, calibrating equipment, training, inspections, and maintaining records are all part of the process controls regimen.

Weld inspections remain a vital and necessary quality component.

However, inspections alone cannot be depended upon to improve weld quality. Whether on a construction job site, in a manufacturing facility, or in a fabrication job shop, the utilization of process controls and nondestructive testing (NDT) methods are crucial parts of an overall QMS. Within a QMS, process validation is required where processes need to be validated before production via a test resulting in

Figure 2. AWS Code D1.2 structural aluminum welds. Welds were rejected due to unacceptable profiles, overlap, and poor workmanship.

STRUCTURE magazine

Figure 1. AWS Code D1.1 structural steel welds on galvanized steel – galvanized coating was not removed prior to welding. Numerous welds were rejected due to unacceptable profiles and poor workmanship.

a Procedure Qualification Record (PQR). Also, appropriate training needs to be provided for welders and all other personnel engaged in activities that affect weld quality. Welders are directly engaged in the “arcs and sparks” and have the most significant impact on weld quality.

If visual weld discontinuities occur during welding, welders bear the responsibility to stop and seek support in determining the root cause as opposed to continuing to weld (Figure 2). To achieve First Time Quality structural welds, control production costs, fabricate safe and reliable structures, employ a well-executed QMS, and continuously maintain welders’ training and qualifications.

Quality Welding and Inspection Weld inspection remains a vital and necessary component of a QMS. The role of AWS Senior Certified Welding Inspectors (SCWI), Certified Welding Inspectors (CWI), and American Society for Nondestructive Testing (ASNT) Level II/III inspectors is to perform weld inspections to determine if the weld meets acceptance criteria of the applicable code, specification, or drawing. Weld inspection is a serious responsibility for all inspectors. For example, although the principal function of AWS SCWIs and CWIs is to perform visual weld inspections, inspectors also review test results of inspections performed by ASNT Level II/III inspection crews. In addition, SCWIs and CWIs review welder qualification test records, welding procedure specifications (WPS), weld maps, drawings, etc., as well as being engaged in maintaining inspection documentation, performing fit-up inspections, verifying weld filler metals, or observing post-weld heat treatment (PWHT).

For a given project, a customer’s weld specification may supersede an industry welding code by specifying more restrictive weld acceptance criteria (e.g., weld toe fatigue blending criteria, beam coping criteria, or workmanship quality). The customer or general contractor sets the tone of a project by conducting pre-project briefings that detail acceptable workmanship quality expectations using photographs, representative work examples, illustrations, and more. The goal of pre-project briefings is to “calibrate” welders, inspectors, quality assurance engineers, and supervisors so pertinent personnel understand the level of welding workmanship quality expected. The customer is the “Final Inspector.”

Hence, it is essential to set the quality standard and establish expectations before project welding, noting that mediocre or marginal quality of welding workmanship will not be acceptable (Figures 3 and 4 ).

Quality of Welding Workmanship The following are key human performance attributes that are contributing factors in achieving premium welding workmanship quality. Ultimately for businesses, the core values and the prevailing paradigm is such that craftsmanship represents a culture, as behavior and a way of thinking.

1) Attitude: Taking pride in one’s personal performance. Being self-motivated to provide solutions to tasks.

2) Accountability: Taking ownership of work/ Figure 3. Bad lighting or paint cannot hide Figure 4. Machined groove weld on an AWS Code performance. Producing and delivering a supepoor, structural steel welding workmanship.

D1.2 structural aluminum weldment. The weld was rior product because the work reflects you. rejected due to cracking and voids.

3) Workmanship: Employing skill/proficiency 6) Professionalism and Integrity: Being honest and trustworthy with which a task is performed – applying and doing what is right in the absence of oversight. Wanting garnered experience. Having zero tolerance for rework and repairs due to poor work practices. to do top-of-the-line work the right way, the first time, or not 4) Teamwork: Collaborating with others to improve project at all (Figure 5, page 14 ). performance. Encouraging an environment of inclusion and constructive brainstorming to improve the overall work plan.

Avenues for Welder Training 5) Knowledge and Training: Seeking opportunities to train and mentor others in the area(s) of your expertise. Seeking There is no substitution for a welder’s proficiency skills and training. self-development via continuous education and training to Proficiency skills are developed and perfected through practice. Some become an expert. companies have established training centers to provide hands-on and ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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access to welding or fabrication instruction. The U.S. military has welding schools, and there are veteran welding training programs accessible by way of companies, organizations, and unions. Select unions, such as the Ironworkers, Boilermakers, and the Pipefitters unions, offer apprenticeship programs that combine classroom instruction with on-the-job training. Regardless of the type of hands-on training, classroom instruction, and teaching received, knowledge is accumulative and never wasted.

The House of Weld Quality

Figure 5. AWS Code D1.6 structural stainless-steel welding craftsmanship: Left and Center: Interpass and cover pass welds (mechanized GTAW) on 1.25-inch-thick plate (1G position). Right: A manual GTAW hot pass (2n d pass) weld bead on a 1.5-inch-thick plate (3G position).

The House of Weld Quality (Figure 6) symbolizes philosophies, principles, and actions essential to quality structural welding and fabrication. The objective is to uphold the structural integrity of the House by not compromising weld quality. The House is constructed on the Foundation of Weld Quality, a QMS such as ISO 9001. A QMS is the House Foundation whereupon the Pillars of Fundamental Welding Knowledge and Workmanship Quality Principles arise. The pillars support the roof, Conformance. Conformance includes the requirements of a project’s Codes, Drawings, Contracts, and more.

The House of Weld Quality is structurally “sound” only as long as the company’s quality system is robust, and everyone involved is vigilant in improving production effectiveness and efficiency through preventing, correcting, and eliminating opportunities for errors, deficiencies, and non-conformances that transpire throughout fabrication. A QMS employs a proactive and predictive process approach (e.g., auditing, reviewing inspection results, and continuously improving the process) versus a reactive approach. Inscribed on the House walls are quality structural welding and fabrication tenets.

Conclusion

Figure 6. The House of Weld Quality symbolizes philosophies, principles, and actions essential to quality structural welding and fabrication.

classroom instruction so that welders can practice and acquire essential skills and knowledge required to work in fabrication departments.

Comprehensive welding schools such as the Lincoln Electric Welding School, the Hobart Institute of Welding Technology, and the Tulsa Welding School are well-recognized schools where welders learn proficiency skills and knowledge. Also, various other private/public vocational and technical schools are available for welding personnel to attend. Welding schools provide hands-on, practical training coupled with classroom instruction where welders can learn the fundamentals of welding as well as associated practices. The AWS Certified Welder Program provides welders with experience using welding procedures employed in structural steel, pipeline, sheet metal, and chemical refinery applications.

Furthermore, AWS offers online courses related to the science, equipment, process variables, consumables, safety precautions, and the advantages and disadvantages of individual welding processes. Online courses are becoming increasingly prevalent in terms of providing

STRUCTURE magazine

Weld defects are not free and come at a burdensome cost. A welder makes weld defects and is being paid for making them, as well as to repair them, and an inspector is being paid to re-inspect for them. Therefore, the importance of comprehensive welder training cannot be overstated. Quality is a business philosophy where the House of Weld Quality exemplifies requisite quality structural welding and fabrication criteria. To achieve quality structural welding and fabrication, a rigorous QMS infrastructure is essential where training, in-process monitoring, process controls, auditing, inspections, and more are aimed at improving production effectiveness and efficiency by preventing, correcting, and eliminating opportunities for errors, deficiencies, and non-conformances that occur throughout fabrication.

What does this mean for you? Understanding how quality can and should be built into a product rather than corrected after the fact can help the practicing engineer better represent the owner, and ultimately, better safeguard the general public. This can be accomplished through development of more quality-focused general notes and specifications, targeting quality in pre-construction meetings, and responding knowledgeably to requests from fabricators or erectors about such quality directives.■ A list of acronyms is included in the PDF version of the article at STRUCTUREmag.org.

William C. LaPlante is a Welding Engineer, SCWI and CWE, in Anchorage, Alaska.

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structural PRACTICES

Transferring Loads in Existing Buildings

By Ciro Cuono, P.E.

E

ngineering schools routinely train young engineers in new systems and materials to prepare them to enter the workforce. However, renovations and adaptive reuse of existing buildings are often overlooked or omitted in an already packed undergraduate schedule. The reality of construction today is that there is a high probability that most engineers will, at some point in their careers, work in some capacity on an existing building. Especially in dense urban environments, or in the older parts of the country with a large stock of existing structures, it is often a better use of resources and more respectful of the environment to reuse and adapt an existing building.

For structural engineers, one exciting and relatively common aspect of renovations and adaptive reuse of a structure is when a design requires the transfer of loads from an existing element to a new element. This can be in the form of SCR or “shore, cut, and reframing” of horizontal gravity framing from an existing element to a new element or transferring vertical gravity loads from a load-bearing wall or column to a new transfer element. These alterations often occur when framing for new stair, elevator, or escalator openings where existing beams are shored, cut, and reframed into new girders. Other examples include 1) cutting openings in existing bearing walls and transferring the load to a new transfer beam to create a larger open space below, and 2) cutting a column and transferring the load to new transfer girders to create a larger opening below or to relocate the vertical elements at the floors below. Adaptive reuse of a building that modifies use and occupancy must take into account a column rhythm that may have made sense in the existing building but does not necessarily make sense with the new use. For example, an office building with a regular grid of columns may not readily lend itself to an exhibition space that

Figure 2. Detail for temporary screw jack set up.

STRUCTURE magazine

requires more significant open space.

Today, the design engineer or engineer of record (EOR) would typically first determine the feasibility of the element to be demolished and design the transfer re-support of the existing elements.

The engineer would then design and detail the new permanent elements and Figure 1. Screw jack set up in preparation for may provide information a shore, cut, and reframe scenario. and direction on the phasing, sequencing, and means of temporary support necessary to install the new framing condition. Due to the prevalent practice of leaving the “means and methods” of construction to the construction team, which provides for a clear delineation of responsibilities, it often becomes the responsibility of the General Contractor to retain their own structural engineer to develop the final design of the temporary works and transfer mechanism, develop a Method of Procedure (MOP) for the work, and oversee the installation of the temporary works.

Guidance for the design of temporary works and load transfer mechanisms is limited. ASCE Design Guide 37, Design Loads on Structures during Construction (ASCE 37), is an excellent resource for computing practical design loads for temporary structures. ASCE 37 provides the minimum recommended design loads for temporary works; however, there may be some uncertainty of actual loads in older pre-war buildings framed with archaic structural systems that then warrant higher factors of safety. Additionally, strength design may not be the governing case, especially in older buildings with brittle finishes that may require strict deflection criteria wherein the strength design is a secondary consideration.

Many practical aspects of this type of work rely heavily on the experience of working with similar structures as well as the knowledge of the fundamentals of structural behavior and the ability to identify and visualize load paths correctly. These skills can be learned from other engineers and experienced contractors who have accumulated this knowledge.

Figure 3. View looking up at a needle shoring scenario.

Figure 4. Needle shoring setup. In this scenario, the needle beams are cantilevered from the inside due to site logistics.

Shore, Cut, and Reframe SCR is a common abbreviation in modifications to existing buildings.

Simple gravity framing that is to be temporarily supported, cut, and then reframed into a new element such as a girder or load-bearing wall can often be supported with adjustable screw jacks with clamps. An adjustable screw jack is a vertical load-carrying member that extends to fit different lengths or heights so it can resolve a variety of situations and can be reused. A typical version of this is a steel pipe column with a threaded bar extender with a wing nut locking mechanism (Figure 1).

Generally, these are intended for light to medium duty installations and vertical gravity loads only. Support at the base is usually with hardwood spreader beams and/or wood cribbing while support at the

top can be with clamps, welding to existing steel beams, or spikes fastened to wood beams. Calculation of the loads on the beam to be supported is routine, and ASCE 37 provides guidance on temporary construction live loads. The existing supporting element(s) at the base can be validated with routine design checks, again, using ASCE 37 for guidance, by temporarily reducing live loads to construction live loads. Spreading the load out on heavy timber cribbing over an area can keep the subsequent pressure reaction to the existing documented live load capacity (Figure 2). In an extensive renovation, where the floor beams being supported have been stripped of finishes, and the area is unoccupied, the shoring can be installed and the existing elements can be cut and reframed to the new supporting elements without concern for cracking finishes. However, where working below occupied spaces,

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Figure 5. View of rear wall with needle shoring. Note the shoring at the window openings.

Figure 6. Needle shoring through a brick wall.

including those with brittle finishes, the adjustable screw jack can be slightly overtightened to provide pressure or pre-loading to the element to be supported, before making the cut, to mitigate deflection and potential cracking. Where lateral stability considerations are necessary, diagonal bracing, using pre-manufactured pipe bracing with clamps, works well with adjustable screw jacks. Where loads are high, or heights are significant, it may be necessary to design custom shores, for example, with hollow structural sections (HSSs). Forged wedge shims can be used to achieve pre-loading.

to a base such as the ground or temporary footing or spreaders (Figure 3, page 17 ). The needle beams, when adequately spaced, support the masonry wall above through the natural arching action of well-bonded masonry. This allows the wall to be safely removed below the needles for the installation of new permanent support elements.

The needle beams should be designed for strength and local stiffness requirements, as well as strict deflection requirements, as mentioned above. The procedure generally requires masons to make small openings through the wall, one at a time, to allow the beam to be pushed or “needled” through. The space around the beam, within the opening, is then packed tight with grout and shims for load transfer (Figure 4, page 17 ). Generally, any window openings above are shored with temporary timbers to maintain an even load distribution and to stiffen the openings so as not to put any undue stress on window elements and lintels, particularly if they are stone (Figure 5).

Since the arching action requires conservative assumptions, like needle spacing at 2 feet to 3 feet on-center, the vertical support columns below the needle beams create a logistical problem for a contractor to bring in the new permanent steel. As a result, the new steel should be fabricated and brought to the site before the full installation of the needle shoring. This allows for workers on the site to not have to meander through a sea of temporary shoring columns, but rather simply raise the new transfer beam since it is already waiting on-site in position for installation. Otherwise, it may be impossible to erect the new permanent beam around the newly installed shoring.

The transfer of load from the wall to the needle beams always creates the risk of cracking brittle materials like masonry even if the beam deflection is held to L/600 on the new permanent support beam.

Where this is a concern, pre-loading of the new permanent beam can be achieved by installing tapered shims between the beam and the wall or, more precisely, via a hydraulic jacking system.

The hydraulic jacking system is a simple set up where portable hydraulic lifting jacks are installed between the new permanent beam and the installed needle beams (Figures 6 and 7). The idea is to pump hydraulic fluid into the jacks, which then extend and impart an equal and opposite reaction above and below. The reaction imparted below to the new permanent beam causes the beam to deflect. The reaction imparted above counteracts the existing weight above. Based on experience, pre-loading to 80 – 90% of the realistic dead weight from above has produced excellent results in the author’s practice. More than this could lift the wall, and less than this may not be fully effective in achieving pre-loading. A good way of handling this is to determine 1) the deflection that the permanent beam would see from the new load, and 2) the resulting pressure in the hydraulic gages.

Needle Shoring Masonry Walls Renovations and adaptive reuses may call for creating new or widened openings in existing stone or masonry walls. Designing a new permanent transfer beam is often the easy part of such endeavors.

For load-bearing masonry, steel beams are typically used for their strength, ease of modification, and combustibility considerations. Per the Masonry Society’s TMS-402-16, Building Code Requirements for Masonry Structures, the deflection of beams providing vertical support for unreinforced masonry should be limited to the span, L, over 600. The trickier part of this design is determining and detailing the phasing, sequencing, and means of temporary support to re-support the wall onto the new permanent beam system. Where new openings are modest in size and the existing wall is robust, such as multi-wythe brick with interlocking header or rowlock courses, it may be possible to chip out one side and install a new steel channel header. This allows the rowlock courses to temporarily transfer the load to the remaining solid half of the wall below; the process is repeated on the other side.

This approach precludes the need for temporary shoring and is simply a sequenced installation of the new transfer beams in a clever way to save the cost of temporary works. This solution is not recommended for long spans, heavy loads, or relatively weak and thin walls.

Another method to achieve a wall opening is to design temporary transfer beams, commonly a pair of channels that are installed on both sides of the wall above the proposed new opening. This is intended to temporarily bridge the load of the proposed new opening so that the opening can then be safely made and the new permanent transfer beam installed. In this method, the wall load is transferred to the temporary channels via adhesive anchors, through bolts in the masonry work, or by cutting the wall out in small sections and installing plates or tubular sections in small lengths to the underside of the channels.

However, for heavier loads and longer spans, the optimal method tends to be needle shoring. Needle shoring is a colloquial term used to describe a series of steel beams that are inserted or “needled” perpendicularly through a wall and then supported vertically down STRUCTURE magazine

Once the jacks are pumped, the pumping would stop when either the deflection or the pre-determined pressure has been achieved. The deflection can be measured via a laser level. The jacks should each be tied to a gage and tied to a central manifold with a gage so that the total pressure and any individual pressure can be monitored. If all goes according to plan, the small amount of actual pre-deflection cannot be detected without a measuring tape or ruler. Once the pre-loading or pre-deflection has occurred, the needle beams can be removed, and the remaining holes can then be bricked solid to the permanent pre-loaded beam, one at a time.

The key to a successful operation is careful, robust preparation. If the work is done well, at the end of jacking, which may only last a few minutes, an anti-climactic moment occurs where nothing happens. For older materials with uncertain properties and uncertain field conditions, the jacking work must occur when no other work is happening. This allows a team member to listen for any cracking noises and continuously monitor the existing wall and framing for any cracks or signs of trouble. A preconstruction meeting, and a checklist for all parties to review, helps to ensure a successful outcome by avoiding mistakes and keeping all parties safe.

Figure 7. Needle shoring in a masonry wall with a hydraulic jack.

Removing Columns Most load transfers in older masonry buildings are generally guided by practical considerations and simple engineering calculations with a heap of conservatism to keep problems from arising. Far from rocket science, the processes described above are familiar to any engineer working on older buildings.

In contrast, removing a column from an existing framed building, usually steel or concrete, is a more complicated endeavor. A scenario such as adaptive reuse, say an older office building being converted to a museum or event space, may necessitate some column removals to achieve a new architectural layout.

The basic concept for the safe removal of an existing column is similar to the needle shoring process with hydraulic jacks. The idea is that a new transfer girder or pair of girders needs to be preloaded and pre-deflected so that when the column below is cut, the pre-loaded and pre-deflected girders want to rebound up. The remaining column above wants to drop down with equal and opposite forces. This cancellation of forces should result in theoretically zero movements, which, in an existing occupied building, would be the measure of success. See Figure 8 for a schematic setup of a jacking assembly.

The division of design labor would follow the usual separation of permanent design by the EOR with the means and methods to be designed by the contractor’s engineer. In a steel-framed building, an effective transfer mechanism is to use pairs of transfer girders that flank the column to be removed. A steel jacket can then be designed around the existing column. The hydraulic jacks are set between the transfer girders and the temporary jacket. This allows the jacking to force the girders down using the weight of the column above as a reaction point.

Each component of the jacket must be checked for the transfer of load.

Conclusion The design of new buildings is basically static. New beams and columns are designed for theoretical loads that may happen at very infrequent intervals, and the building is erected from the bottom up. Transferring loads in existing buildings as a result of cutting walls and columns is inherently a dynamic process since the load must transfer from one element to another. In general, the statics and structural analysis are rudimentary; however, planning and

Figure 8. Schematic section at column jacking assembly.

method-of-procedure are critically important. Control of deflections through pre-loading is a simple and effective way of dictating how the structural components are to behave to avoid sudden movements and unwanted cracking.

Similar to other aspects of a construction project, the design, detailing, and follow-through for the means and methods of transferring load is a team effort that requires planning and communication with all parties. Perhaps even more than in new construction, a shared understanding of the end goal combined with ongoing collaboration between the designer, the contractor, and the contractor’s specialty sequencing/means and methods engineer is the key to success.■Ciro Cuono is the Founding Principal of Cuono Engineering PLLC, a structural engineering firm located in Port Chester, NY, and NYC, and is a past Assistant Adjunct Professor at The Bernard and Anne Spitzer School of Architecture at the City College of NY. (ccuono@cuonoengineering.com) NOVEMBER 2020

Shoring the Unshorable Challenges in Repairing a Severely Corroded Rooftop Cooling Tower Support Structure

By Roya A. Abyaneh, P.E., and Edward S. Breeze, P.E.

R

epair of corroded steel supports caused by a leaking cooling tower on an aging building’s roof can be incredibly challenging for structural engineers.

A case study of rooftop cooling tower support structure repairs highlights common obstacles and solutions for these rising problems.

Deteriorated steel structures are often repaired by reinforcing the existing elements with additional steel. In cases of severe deterioration, replacement may become more practical than reinforcement. Full or partial replacement of the structural support requires unloading the support by temporarily shoring its load. This may seem a straightforward operation, but it can quickly become challenging when it involves shoring a 350,000-pound cooling tower on the roof of a high-rise designed for a modest 20 pounds-per-square-foot live load. Although it may be tempting to postpone these complicated repairs, the cooling system’s failure could make the building uninhabitable – or worse, if the support structure collapses onto the fully-occupied office space.

Case Study Figure 1. Corrosion of steel beams below the cooling tower.

Figure 2. The base isolator bearing plate was temporarily welded.

The case study involves a cooling tower on top of a 46-story commercial building constructed in 1975. The building was well maintained, but care of the rooftop structure was deferred. In 2018, Engineering Diagnostics was engaged to design repairs for corrosion of the cooling tower support structure discovered after a change of building maintenance staff.

The cooling tower serves over one million square feet of office space in a hot and humid climate, which means that it had to remain operational during the repairs.

The approximately 2,400-square-foot cooling tower is a timber and fiberglass structure supported by steel wide flange beams and girders that bear on base isolators.

Some of the steel shapes used in 1975 are no longer produced, requiring research into the American Institute of Steel Construction’s (AISC) archives of “historic” shapes. The base isolator springs dampen the cooling tower’s mechanical vibration before transferring the load to concrete pedestals that support the tower on the roof.

The existing structure’s assessment revealed that the steel girders had experienced up to 50% section loss in some of the webs and flanges. In some locations, personnel could poke their fingers through the web (Figure 1). The most severe corrosion was observed directly over a central concrete pedestal. Each of the 12 concrete pedestals was built with four encased steel pipes, which had also been damaged by corrosion.

The preliminary repair strategy consisted of (1) partial replacement of the corroded girder, (2) reinforcement of less corroded beams with angles and stiffeners, and (3) patching spalled concrete on the pedestals. Temporary shoring was installed while Engineering Diagnostics worked on the development of the final repairs and shoring plans.

Design Challenges

Figure 3. Post shores and horizontal shoring beam.

STRUCTURE magazine

At first glance, the solution for reduced steel cross-section is simple: add more steel. The challenges posed by the cooling tower’s location, its significant weight, and minimal roof capacity required several iterations to the repair strategy. The original design drawings were illegible; therefore, the as-built construction had to be visually confirmed.

The building maintenance staff reported the tower was operating with 60% more basin water than specified in the original design documents. The weight of water was a particular concern for the performance of the base isolator springs and the temporary shoring.

The steel support structure had been wetted by rain and continual leakage from the cooling tower basin. In addition to steel corrosion, the base isolator springs

had bottomed out from age and overload. This resulted in vibrationinduced spalling in the concrete pedestals.

The corrosion across the web and flanges of some sections was so significant that it would be necessary to cut out the deteriorated length and to splice in new pieces. Adding steel plate reinforcing was also considered; however, several areas exhibited insufficient “healthy” steel for suitable welding or composite shear flow through the beam.

Further, the repair steel would have covered most of the existing steel, impeding the ability to monitor for future corrosion.

Substitution of the damaged beam section appeared more practical.

Shoring would need to redirect the cooling tower loads while the corroded steel was cut out, and a new section was spliced in. Unloading the members nearest to one pedestal by shoring would result in a change in the loading of the other pedestals. The roof framing was analyzed as a system to understand the unintended effects of shoring.

Since the floor below was occupied, extending the shoring to lower floors was not a viable option. Congestion of pipes and air ducts below the tower further constrained the shoring options. RISA-3D was used to develop a detailed three-dimensional model of the cooling tower support structure and roof framing. The model was used to identify the shore positions such that the members of concern were completely unloaded, and the roof framing members and connections would not experience a surcharge. Shoring beams were placed below the cooling tower and on the roof deck to transfer the loads directly to a strong roof beam rather than the slab spans.

Construction Challenges The first stage was to remove the rust, and that is when construction obstacles began. A sandblasting compressor could not be brought to the roof, so the rust had to be removed by grinding, which extended the cleaning time. The roof hoist’s elevator dimensions and capacity limited the weight, transportation, and assembly of tools, shoring, temporary beams, and repair materials. Steel cleaning near the building air-intakes required careful protection to ensure tenant safety. The cleaning process revealed additional steel damage and section loss, which necessitated additional iterations of the repair design.

The structural evaluation of the roof framing indicated that only portions of the cooling tower could be shored at one time to avoid overstressing the framing with concentrated shoring loads. Partial shoring increased the risk of differential movement and subsequent seam leaks in the basin’s plywood decking. The cooling water leakage risk applied to potential machinery damage, water infiltration to the commercial space below, and continued corrosion.

The cooling tower basin consists of plywood sheets supported on 4-inch square (4x4) wood beams supported by the steel beams and girders.

Unloading the steel beams required using post shores to lift the wood beams just enough to remove the corroded steel beams while minimizing the differential movement in any zone. It was essential to schedule hold points during the shoring process to monitor for new leaks.

Before the shoring process began, the as-built construction of the roof framing was verified by accessing the roof ’s underside. Temporary lateral braces were installed at shoring points and at locations where bracing members were disconnected. Although the lifting process was intended to unload the steel beams, it would also decompress the isolator springs supporting them. This decompression would push the beams upwards and prevent the intended clearance gap between the steel beams and the cooling tower. The base isolator’s top bearing plate was temporarily welded to lock its position (Figure 2). Shoring beams placed on each side of the repair lifted the wood beams from the top of the subject steel (Figure 3). A laser level was used to monitor the displacement progress.

The post shores operate based on a screw jack system.

Once the shores were each snug-tight against the structure, the lifting team would advance all shores in a row by half a turn, alternating between rows. Lifting the wood 3⁄16 inch proved adequate to install the new steel beam sections.

After stabilizing the shores, the contractor cut out the deteriorated beam section, Figure 4. Repair of the concrete pedestals and a new piece was spliced revealed the encased steel pipes. with welded full-moment connections. The placement of welded splices was designed around the wood beam locations with appropriate fire watches. Bent splice plates were used to join the new and old steel. These plate bends were necessary because of the mismatch between the historic steel shape heights and the new steel shapes.

The remaining beams and girders were repaired by reinforcing with bolted composite steel angles at the flanges and adding web stiffeners.

The cooling tower did not need to be lifted for the reinforcing repairs because the beams were not replaced. Shoring was used to unload the steel sufficiently to allow the reinforcing to share the load of the tower, as opposed to carrying incremental additional load.

Because of the wet environment, it was essential to prevent water from being trapped between the new and old steel. The perimeter of the steel joints was sealed, and the structure was epoxy coated to ensure the durability of the repairs.

The concrete pedestals also revealed more significant distress than initially evident. The contractor removed loose concrete up to 8 inches deep, revealing encased steel pipes (Figure 4 ). Ultrasonic testing was used to confirm that the pipe thickness was adequate to remain in place. The steel was cleaned, new mild steel reinforcement was installed, and concrete patching was placed.

Conclusions and Recommendations What initially appeared as a straightforward steel reinforcing project revealed several interesting challenges during the structural assessment and the subsequent repairs. This project’s successful execution was made possible by timely and clear communication between the property management team, contractors, and engineers.

The repair of rooftop cooling tower support structures can be risky due to unique challenges. In the case study, these included building occupancy, the cooling tower assembly’s sensitivity to displacement, large supported loads, low roof capacity, and lack of original design documentation (typical for older buildings). Consistent monitoring of the process by the engineers was critical to ensure that the design intent was met, especially in rehearsing the shoring plan. This project had easy access to visual monitoring of the health of the structure.

In the end, monitoring and maintenance is the best course of action to prevent and mitigate the need for such repairs and preserve the integrity of older buildings.■ Roya A. Abyaneh is a Senior Engineer in the Houston, Texas, office of Engineering Diagnostics, a d/b/a of Building Diagnostics, Inc. Edward S. Breeze is a Principal and Branch Manager of the Houston, Texas, office of Engineering Diagnostics.

NOVEMBER 2020

INNOVATIVE

Design

Using Composite Steel Joists 121 SEAPORT BOULEVARD

By Michael Martignetti, P.E.

Innovation is evident in the Boston skyline. A talented group of architects and engineers have designed the structures built in the last 30 years in Beantown. These creative firms continue to incorporate new design concepts that meet their client's needs. Clients get a great, sustainable structure that utilizes the best materials and the most economical design. The public – well, they get to look at and utilize the buildings. One engineering firm has done its share of design in Boston: McNamara·Salvia Structural Engineers.

O

ne structure of particular note is the recently completed 121 Seaport Boulevard in the Seaport District of Boston.

The McNamara·Salvia (McSal) team, under the guidance of Principal John Matuszewski, P.E., worked with CBT Architects on this 17-story, 440,000 square foot, Class-A office building. Per McSal’s website, “121 Seaport’s unique design is carefully choreographed to promote innovation and collaboration, and increase employee productivity in the workplace.” The building features a three-story lobby, dedicated fitness center, views of the harbor and skyline, and wide-open office space to meet the changing requirements for commercial office layouts. The structure’s elliptical shape avoided underground tunnels and zoning issues and reduced the lateral wind load on the aerodynamic structure.

This, in turn, reduced costs and allowed for a very open concept with minimal columns and floor-to-ceiling glass windows. The open concept utilized plenty of natural light and helped the building achieve LEED Platinum Certification. Top-down construction meant that the superstructure was built while the underground parking garage was simultaneously excavated. This added to the design challenge, but ultimately saved millions of dollars and six months of schedule.

At the core of this beautiful building are CJ-Series composite steel joists. McSal has used CJ-Series joists successfully in several midrise buildings throughout Boston. CJ-Series steel joists support the composite steel deck and the concrete slab poured on top. Similar to composite beams, shear studs are welded through the deck and to the joist’s top chord to develop shear transfer from the joist to the concrete slab. The composite action between the joist and concrete allows the concrete to act as the compression chord element of the joist. As the slab works compositely with the joist, the moment arm or distance STRUCTURE magazine

between the tension force in the bottom chord and the compression force in the top chord increases. This reduces the required angle sizes in the joist chords, which lightens the overall joist weight and cost.

Composite steel joists offer several advantages compared to composite beams. The open web steel joists can span long distances to create wide-open spaces. At the same time, they offer an excellent span-toweight ratio, which decreases the overall weight and cost of the steel structure. A maximum span-to-depth ratio of 30 is greater than the typical roof joist span-to-depth ratio of 24 and allows even shallower joists to achieve the wide column spacing. A typical 22-inch-deep composite joist spanned anywhere between 30 feet and 48 feet on the project, accomplishing the elliptical shape. The joists were spaced 10 feet part given their strength and were topped with 2-inch composite deck, ¾-inch-diameter shear studs, and a 5¼-inch lightweight slab.

Camber in the joists was designed to offset construction dead load deflection. The efficient design reduces the overall number of members required, speeding up the erection of the structure. Minimizing the joist count also reduces the amount of spray fireproofing that must be done to achieve the required fire rating for the floors. The open web construction of the joists allows MEP trades to run plumbing, electrical, and mechanical equipment through the joists rather than under the beams. This minimizes the overall floor depth, maximizes ceiling heights, and/or reduces overall building height.

The office tower’s wide-open spaces created a need for extensive ductwork to heat and cool the spaces. Vierendeel openings were created at the midspan of the joists to accommodate the ductwork. Vierendeel openings are designed by the joist manufacturer, eliminating joist webs for a section of the joist so that there is an unobstructed space

between the top and bottom joist chords. behavior of the building with this shape, The joist’s midspan is an ideal location as combined with varied member lengths, the shear forces are significantly less at this made the analysis complex. Ultimately, a point of the joist. The Vierendeel openings single minimum moment of inertia was were strategically aligned from joist to joist provided for the joist designer to keep to create a wide-open passage where all of the design simple while ensuring that the the ductwork was concentrated, avoiding lightweight members met the vibration putting anything below the joists. With requirements. all of the MEP organized within the joist Cives Steel Company – New England envelope, the structure afforded the fit-out Division, the structural steel fabricator, that architects desired with the ability to and James F. Stearns Co, the steel erector, leave the ceiling exposed to view. constructed the main steel superstructure Given the goal to minimize the floor enve- Shear studs welded through the deck to the top chord create the over a period of six months. When asked lope, McSal presented the concept of flush composite action. about the composite joist design, Brock frame connection to attach the composite Bessey, Project Manager for Cives, stated, joists to the girders. The joist engineer and “Composite joists worked great for this steel connection designer then coordinated project due to the need for the design to finalize this connection’s design details, team to pass the MEP trades through the which offered several advantages. Typically, support members. Creating openings in open web steel joists sit on top of the girda wide flange beam would require a good ers with 5-inch-deep seats. By dropping the deal of shop reinforcing, especially for joist between the girders, that 5-inch depth large web penetrations.” The joists were was not added to each floor envelope. The connected directly to the concrete core flush frame connection design allowed the at the center of the building and varied erector to utilize a bolted connection, sigin length at the perimeter to create the nificantly reducing the amount of welding “football-shaped” structure. Soft shoring in the field. was placed at the edge of each side of Open web steel joists allow MEP to easily route through them to Additionally, the overall length of the minimize floor envelope. the opening to maximize the size of the joists was shortened, making it easier to vierendeel opening while minimizing the maneuver the joists into place. The flush frame connection meant joists’ weight. Soft shoring means that the shoring posts are set to a some additional shop work for the production of the joists and girders, height that allows the joists to deflect to the elevation that results in but this cost was controlled in the shop environment. Ultimately, the a level slab of consistent thickness due to the self-weight of the slab ease of erection saved both time for the overall construction schedule before supporting the midspan of the floor until the concrete cures and cost for the superstructure. and composite action ensues. For this project, the shoring was only Bryan Hilton, P.E., Senior Project Manager for McSal, served as a needed for three days, allowing construction to move quickly. key member of the design team throughout the project. When asked 121 Seaport Boulevard has left an imprint of innovation on the about why composite joists were chosen for the project, Hilton com- Boston skyline with its curves and creative design. It offers its tenmented, “the selection of the composite steel joists with composite ants state-of-the-art open spaces full of natural light and flexibility by beam girders was the optimum framing system for the project because utilizing composite steel joists rather than a typical composite beam of its long-span capability, the economy of weight, enhanced flexibility design. This was all done while minimizing the structure’s weight and with the building’s MEP system, reduced floor-to-floor height, and utilizing an innovative construction sequencing saving both time and its successful performance in past projects.” The McSal team worked money for the project. Looking to the future, this project hard to optimize the benefits that the composite joists offered for the sets a new bar that will challenge designers to continue to structure. Hilton stated, “a design challenge was the incorporation of innovate in the years to come.■ a simplified and comprehensive approach to specifying the composite Michael Martignetti is the Vice President of Sales for Canam Steel Corporation, steel joist’s required moment of inertia to satisfy the building’s office a Steel Joist Institute member company. (michael.martignetti@cscsteelusa.com) vibration criteria given the elliptical shape of the floor plate.” The

Bolted flush frame connections for joists to girder.

Vierendeel openings at midspan allow for a large chase of ductwork.

NOVEMBER 2020

structural SYSTEMS Lightening the (Dead) Load on Floors

By Tim Liescheidt, P.E.

I

nnovation can be a wonderful thing. For engineers, creating, innovating, or even just utilizing others’ new ideas can benefit a client and a project. And many engineers would say that doing something new is almost always more

fun than repeatedly reusing old ideas or methods.

Figure 1. Comparison of typical composite joist section cut and typical CFS truss section cut.

For some engineers (the author included), construction using ColdFormed Steel (CFS) materials fits into this idea of innovation. CFS is a relatively new product line that continues to find more and better applications in the world of construction. The use of CFS in floors is a perfect example.

Technically, CFS has been around for over a century. Since its first use in basic structures and small homes in the 1850s, its use over the next 140 years seems to have been sporadic at best.

Despite the slow start, the use of CFS has grown exponentially over the last 30 years. There are probably very few, if any, architects, engineers, or building officials left who are not at least somewhat familiar with its use. Since the late 1990s, a better understanding of CFS’s versatility and the development of design standards have allowed engineers to specify CFS on projects confidently. The American Iron and Steel Institute (AISI), along with countless engineers, designers, and researchers, have developed and made available CFS standards for use in building design. Technical Notes have been written to address common questions. A few examples of the topics addressed in these Tech Notes include, but are not limited to, avoiding corrosion in CFS members, attachment of CFS to other materials, and design of lateral load resisting elements.

Additionally, multiple software packages now exist to help engineers design anything from the smallest building component to a full CFS load-bearing structure. In short, design resources that were not available previously for CFS products are now readily accessible to assist the design community.

structures, with 10 stories being a widely accepted ‘maximum height.’

However, since CFS is non-combustible, that 10-story height limitation is not due to code; it is only limited by design.

In 2016, the Steel Framing Industry Association (SFIA) commissioned a study to evaluate how high CFS framing could go.

Patrick Ford, P.E., of Matsen Ford Design in Waukesha, WI, a very well-known and respected engineer in the CFS design community, developed the SFIA Matsen Tower, a 40-story CFS framed high-rise!

The design was even considered ‘conservative,’ and 48 stories has been mentioned as a more accurate limitation.

So how did Patrick Ford develop a design four times taller than the perceived limit? One significant factor was lightening the dead load supported by the walls. A residential structure, such as the theoretical ‘Matsen Tower,’ will need to be designed to support floor live loads of 40 psf. Subsequently, the load-bearing walls need to be capable of carrying this load to the foundation. If a typical floor system, like one using structural steel and concrete, has a dead load of 45 psf, the amount of total cumulative load carried down to the foundation is significant. It is easy to see why

Limitations of Cold-Formed Steel Projects using CFS as the primary load-bearing material are prevalent in midrise construction and most frequently seen in 5- to 10-story STRUCTURE magazine

Figure 2. UL H524.

CFS construction is limited to 10 stories using this example. The Matsen Tower achieved 40+ stories by using lighter materials for floors…specifically Cold-Formed Steel.

To be a more widely accepted alternative for floors, more versatile CFS systems designed for this application needed to be developed that include the substrate in addition to the support instead of using CFS products as the support for existing substrates, such as poured concrete slabs.

The System Structural concrete (SC) panels have gained popularity over the past several years as a lightweight alternative to poured-in-place concrete slabs. Depending on the brand of SC panel used, they weigh approximately 5.3 psf and, by significantly reducing the dead loads of the floor, increase the effectiveness of the CFS products supporting them. Much like plywood that is used in wood framing, they come in 4- x 8-foot sheets and are usually ¾-inch-thick.

These panels have the strength of concrete, are lightweight, and are non-combustible. Like plywood, these panels may be cut with a circular saw and are screwed to the supporting CFS members.

Under standard residential floor load, structural concrete panels require support at a maximum of 2 feet on-center., which corresponds well with the requirements of CFS Trusses and closely matches typical load-bearing CFS stud spacing. For a 26-foot span, a CFS truss with a structural concrete panel floor will have a dead load of only 8.4 psf with an overall depth of 12.75 inches (not including the ceiling).

Used together, CFS framing and SC panels can result in an efficient and cost-effective floor system and another portion of a structure that can be designed using efficient CFS construction. The lightweight CFS member

Figure 3. CFS steel floor truss with a structural concrete panel.

supporting the non-combustible lightweight structural panel combine to create an assembly that can be used in place of structural steel and concrete. Spans are comparable to those of composite steel joists/concrete slabs and do not increase the overall depth of the floor system (Figure 1).

Additionally, by saving over 35 psf in floor dead load versus composite joists with decking and concrete and 60 psf versus hollow-core plank, this lighter weight floor system may significantly reduce foundation requirements. The ability to design a lighter weight structure without sacrificing floor depth, load-carrying capacity, or safety is something building engineers often strive for. Building contractors always appreciate systems that allow them to build lighter-cheaper-faster.

continued on next page

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This system has been tested and awarded a 2-hour unrestrained fire rating by UL (Figure 2, page 24 ). In at least one instance, the test only ended due to heat transfer through the floor covering; the trusses remained intact, supporting the full design load. Additionally, it can be installed by the same trades doing the load-bearing CFS wall framing, reducing the potential for coordination issues due to multiple trades required for the framing system.

Designing

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Like wood floor truss systems, designing with CFS Truss components is specialized and not something that a building designer should be burdened with. Also, like wood floor trusses, specialized designers and software packages make it very easy for truss manufacturing companies to provide the design of the individual components that make up the CFS Truss portion of the System. A building designer can simply specify loading, spacing, and allowable truss depth, and a truss designer can determine member sizes within the truss components to work for the given criteria. Span tables provided by truss manufacturers can give a building designer basic guidelines for load capacities at specific spans and depths.

Structural Concrete panels attached to CFS Floor Trusses need to be fastened to provide adequate diaphragm strength (Figure 3, page 25).

The major board manufacturers have done comprehensive testing on their products and have published diaphragm capacities for systems utilizing CFS Trusses and SC panels.

From a design standpoint, it is important to note that CFS Trusses have been used for 30+ years and were initially developed for use in roof truss applications; CFS Trusses in floor applications are more effective and efficient when using CFS Truss Sections that are explicitly designed for that purpose. For example, diaphragm capacities of SC panels are commonly limited by the fastener edge spacing attaching the panel to the truss member. A traditional CFS Roof Truss product will not have adequate flange width to allow required fastener edge distance. Several Floor Specific CFS Truss products have recently come to market considering this limitation and account for this in the truss section’s design.

In seismic applications, the floor system’s required diaphragm capacity can be significantly less with a CFS Floor Truss System versus structural

Eventually There is a BETTER idea...

INTRODUCING

ADVANT

 LIGHTER than traditional floor systems  FASTER installation  LESS COST versus traditional bar joist/ concrete and existing Cold Formed Truss Systems

STRUCTURE magazine

steel and concrete alternatives. The large reduction in dead load using CFS Floor Trusses with a relatively light Structural Panel can conservatively cut story shear in half as it is collected through a structure.

Sound Attenuation Depth plays a critical role in the design of CFS Trusses. In general, the deeper the truss, the lighter the members. It turns out depth plays a significant role in sound reduction; the deeper the joist or truss member, the less sound transfers between floors. Since trusses can be fabricated deeper than standard joists, a truss may be a quieter option. The spacing of supporting members will also have an effect because sound may be lessened by ‘decoupling’ or creating a separation between the sound and mass in a floor. A wider cavity, or space between joists/trusses, results in a more significant ‘buffer’ or separation between sound transferring members. This is even more advantageous if insulation fills the cavity between joists/trusses. Again, this makes using CFS Trusses beneficial compared to CFS joists.

Since trusses typically have more significant load-carrying capacity, they may be spaced further apart than the more shallow CFS joist, increasing the decoupling in a floor system. Incidentally, this also results in fewer pieces to install and easier access for mechanicals.

And finally, for improved sound attenuation, it is best to screw the subfloor to the truss/joist support away from the truss/joist web, in other words, as far out on the flange as possible. CFS Truss Chords designed for Floors have a distinct advantage over traditional CFS Trusses used in Roof Construction. The wide flange of these Floor Truss members allows the subfloor to be attached farther from the web, further deadening sound.

History Repeats Itself

It was the 1950s when drywall products were gaining broad acceptance in wood-framed construction but still were not a good option to replace plaster over CMU walls in non-combustible structures. At the time, the United States Gypsum Corporation (USG) had proven the fire resistance of its wallboard products. However, to be successful in non-combustible buildings, they needed a non-combustible substrate to attach to. After researching possible options, CFS proved to be the ideal material, and the Cold-Formed Steel stud wall was born. Fast forward to 2020, and hisU.S Patent No. 10,570,618 tory is repeating, including some of the same players. USG is one of just a few companies that have developed a lightweight, non-combustible floor substrate product. For this floorboard product to be successful, it needs a lightweight, non-combustible, strong, and versatile support. Once again, CFS is proving itself to be the ideal material.

Finally, there is an option for the design community to use Cold-Formed Steel in floors as part of a Cold-Formed Steel structure. It was a long time in development and required collaboration from many product manufacturers – but it will prove worth the wait!■

www.advantsteel.com

Tim Liescheidt is an Engineer for Advant Steel LLC. (tim@advantsteel.com)

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community RESILIENCE

A New Challenge to the Practice of Structural Engineering

By Bruce R. Ellingwood, Ph.D., P.E., N.A.E., F.SEI, Dist. M.ASCE, John W. van de Lindt, Ph.D., F.SEI, F.ASCE, and Therese P. McAllister, Ph.D., P.E., F.SEI, F.ASCE

D

espite the progress that has been made in disasterrelated science and technology and the significant

financial investments made at the federal, state, and local levels in risk mitigation, The National Academies continue to view community resilience as a national imperative. Events such as Hurricane Katrina in 2005, the Christchurch earthquake and Joplin tornado in 2011, and Superstorm Sandy in 2012 highlighted the need to better understand community resilience to mitigate the risks associated with severe natural hazards.

Figure 1. Stages of community resilience.

Investigations of recent disasters have revealed the importance of resilience planning for integrated community performance and functionality following disruptive hazard events, including response and recovery, rather than merely addressing public safety and the “sunny day” functionality of utilities. Resilience is defined as the ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions caused by natural and man-made hazards. The spectrum of resilience modeling and assessment requirements for community physical, social, and economic systems is summarized in Figure 1.

any consideration of structural repairs or functional recovery when evaluating design options.

Codes, standards, and building regulations in the U.S. have been developed by different professional organizations and agencies, with variable performance objectives for life safety relating to hazard events, service periods, reliability, and recovery of function. A critical aspect of community resilience – a community’s social needs and objectives, especially concerning post-disaster recovery – is not reflected in codes and standards and other regulatory documents that are used to design individual facilities.

Codes, Standards, and Building Regulations

Achieving Community Resilience

A community is defined as a place (such as towns, cities, or counties) designated by geographic boundaries that functions under a governance structure for decision making. A community has a built environment and social and economic systems to provide essential community needs, such as shelter, transportation, power, potable water, sanitation, employment, commerce, education, healthcare, and government.

Community resilience addresses the ability of a community’s buildings and infrastructure systems to deliver these essential community needs reliably and at a reasonable cost, both before and within a specified timeframe after a damaging event. The integrity of the built environment is central to the resilience of a community’s infrastructure and social and economic institutions. Still, there is currently no common technical basis for linking community-level perTable of examples of community performance goals and resilience metrics. formance goals with performance objectives of Community Performance Goals Resilience Metrics codes and standards for the design of individual Population Stability Dislocation and migration; housing availability buildings, bridges, and lifeline systems.

Change in employment, taxes, and revenue Economic Stability (resources) and community budget (needs) Measuring The performance of civil infrastructure, which is essential to community resilience, is, first and foremost, the responsibility of structural and civil engineers. Codes and standards largely determine the performance of civil infrastructure (e.g., International Building Code, ASCE Standard 7, AASHTO Bridge Design Specification). These codes and standards for buildings and bridges apply to individual facilities. They are focused on life safety goals because of the nature of the building regulatory process. Except for structural systems assigned to ASCE 7 Risk Category IV, the role of individual buildings in fulfilling community resilience goals seldom is recognized by codes. Nor is there

Social Services Stability

Access to healthcare, education, retail, and banking

Physical Services Stability

Functionality of buildings, transportation, water, wastewater, electric power, gas, and communications

Governance Stability

Access to police and fire protection; essential public governmental services

STRUCTURE magazine

Community Resilience

A new, interdisciplinary approach is required to achieve community resilience goals, one that addresses the interdependencies among the physical, social, and economic systems on which a

healthy and vibrant community depends. The Center of Excellence for Risk-Based Community Resilience Planning (the Center), headquartered at Colorado State University and involving 13 partner universities, was established by the National Institute of Standards and Technology (NIST) in 2015 to advance measurement science for community resilience. This includes identifying key resilience factors and metrics, assessing the likely impact of natural hazards on community functions, and providing decision support through risk-informed options with optimal strategies for improving resilience.

Community-level performance goals are often stated as long-term aspirations for the functionality of physical, social, and economic systems. Designers need quantitative performance objectives and design criteria for the evaluation of individual facilities and systems that can support community goals., The collective performance of infrastructure systems and facilities must be quantified using metrics related to functionality and recovery to link the response to community resilience goals. The development of community metrics is a critical aspect of the cooperative partnership between NIST and the Center for addressing community resilience on a national scale. The Table gives a few examples of community performance goals and associated resilience metrics being used by the Center.

A Computational Toolbox

The initial (December 2019) release is available at https://bit.ly/2UllQBe or https://bit.ly/3h9hV3V and has example case studies for communities subject to an earthquake, tsunami, and tornado events. Technical support and user manuals are available, as well as the opportunity to join user groups.

Additional releases will be made as features are developed. Future IN-CORE releases will include a recovery module to capture the process of community recovery to a specified state of functionality. Also included will be a decision/optimization module, which will use selected characteristics of physical, social, and economic infrastructure systems to identify optimized strategies for pre-hazard mitigation and post-hazard recovery.

The Role of Structural Engineering Structural engineers and structural engineering technology have a significant role in improving the resilience of a community to natural hazards because the performance of the built environment is central to the welfare of any community. Public safety aspects (such as stability and life safety) are addressed by either conventional building codes or through novel performance-based engineering (PBE) approaches. In

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One of the Center’s significant accomplishments in its first five years has been the development of a modular, open-source computational environment, IN-CORE – Interdependent Networked Community Resilience Modeling Environment. IN-CORE allows researchers, engineers, and community planners to simulate the impacts of natural hazard events on communities and the recovery of community-wide functions to evaluate and optimize alternative resilience enhancement strategies in support of community planning.

IN-CORE is a multidisciplinary computational environment, with supporting databases, which models community systems through a set of modules and algorithms. IN-CORE is structured so that users can utilize core modules or include their own analysis modules, depending on their needs. The core IN-CORE algorithms for modeling interdependent physical, social, and economic systems, that are included in the initial release, include: • The hazard module includes algorithms for earthquakes, synoptic windstorms, tornadoes, hurricanes, wildfires, tsunamis, and floods.

• The physical infrastructure module includes buildings, transportation, water, energy, and telecommunication systems. Fragilities (for discrete elements) and repair rates (for line elements within networks) predict the physical damage, potential repair rates, and recovery times for a given hazard scenario.

• The social and economic modules currently predict population dislocation, housing repair and recovery, and business interruption and recovery. Changes in the local economy and demographics are based on post-event functionality of the physical infrastructure of the community and a computable equilibrium model of its economy.

Figure 2. De-aggregation of community resilience goals for structural design.

NOVEMBER 2020

contrast to building code approaches, PBE can enable risk-informed design and decision-making for innovative structures with performance objectives beyond code requirements.

In current PBE of buildings, the performance objectives are established for occupant needs and/or building functions (e.g., data centers, specialty manufacturers, hospitals). The role that buildings play in the resilience of the community seldom is considered. Many aspects of resilience, including loss of functionality and recovery, require more comprehensive and standardized assessment methods.

To advance the current state of practice, the structural design process should start with individualized community-level goals to inform performance objectives for individual projects, including recovery-based performance objectives. Community performance goals and metrics must be de-aggregated to the individual facility level. Then, goals and metrics can be used to develop risk-informed design standards and guidelines, code approaches, or PBE criteria that can be used by structural engineers and other design professionals. This process is illustrated in Figure 2, page 29.

A Path Forward

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Best practices of design professionals and decisions by city planners and regulatory authorities are likely to evolve in the coming decade to support community resilience. However, buildings, bridges, and other civil infrastructure facilities will probably continue to be designed on an individual rather than a community basis.

PBE provides a path forward for addressing this conflict and resolving the inherent challenges that will arise in solving both facility and community needs. These challenges will likely require a fundamental change in the way that code- and standard-writing groups approach their tasks to achieve shared community resilience goals. A few of these challenges for structural engineers include: • A broadly based stakeholder group should identify common community resilience goals; traditional performance measures are not sufficient to ensure community resilience.

• Methodologies are needed to guide the development of community resilience goals and metrics and to derive performance objectives and design criteria for individual projects.

Otherwise, there will be no consistent basis for national or regional building practices because resilience goals and metrics for each community are unique.

• Performance objectives for buildings by functional categories or groupings (e.g., residential buildings, commercial facilities, government) or socioeconomic institutions (e.g., education, health care) should be expressed as requirements that are compatible with engineering practice and practical to implement from an engineering perspective.

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• PBE to support community resilience must acknowledge the reality of the U.S. building regulatory process, which is likely to change slowly.

• Reliability targets for individual buildings in current structural design practices (e.g., ASCE 7-16 Section 1.3) set minimum performance requirements at the component level for most design loads except earthquake loads. Target reliability and performance criteria at the system level for all loads are needed to support community resilience goals.

• Codes, standards, and regulations for infrastructure systems (e.g., buildings, bridges, lifelines) should be coordinated to support community resilience goals and to address the functionality and recovery of civil infrastructure as well as life safety.

• Planning and development of major projects will be increasingly performed by interdisciplinary teams, which must be able to communicate aspirations, goals, metrics, and risks to one another. This shift will require changes in university education, especially in civil engineering, to ensure that these teams can effectively work together.

Conclusions The structural engineering profession has a central role to play in enhancing community resilience because the built environment is fundamental to a resilient community. The Center recently has been renewed for 2020-2025, with a focus on community engagement and risk-informed decision-making with the ultimate goal of informing the development of efficient decision support algorithms that are useful and useable for community resilience planning. Future releases of IN-CORE are planned at approximately 6-month intervals with a critical focus on supporting decisions and implementation, as the Center continues to advance the science of community resilience.

The structural engineering community is invited to participate in this exciting new endeavor by exploring the Center webpages for information at http://resilience.colostate.edu.■

Acknowledgment Funding for this study was provided as part of Cooperative Agreement 70NANB15H044 (2015-2020) and 70NANB20H008 (2020-Present) between the National Institute of Standards and Technology (NIST) and Colorado State University. The authors acknowledge the numerous researchers and students working on behalf of the Center; a full listing can be found at the above website. The content expressed in this article are the views of the authors and do not necessarily represent the opinions or views of NIST or the U.S Department of Commerce.

References are included in the PDF version of the article at STRUCTUREmag.org.

Bruce R. Ellingwood is Professor and College of Engineering Eminent Scholar and Co-Director, Center for Risk-Based Community Resilience Planning, Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO. (bruce.ellingwood@colostate.edu) John W. van de Lindt is Harold H. Short Endowed Chair and Co-Director, Center for Risk-Based Community Resilience Planning, Department of Civil and Environmental Engineering, Colorado State University Fort Collins, CO. Therese P. McAllister is the Community Resilience Group Leader and Program Manager, Materials and Structural Systems Research Division, the Engineering Laboratory (EL) at the National Institute of Standards and Technology, Gaithersburg, MD.

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OUTSIDE the box Utility-Scale Photovoltaic Power Plants Analysis and Design

By Sumanth Cheruku, P.E., and Matthew T.L. Browne, M.Eng, P.Eng, M.ASCE

R

enewable power generation nearly doubled in the past decade, growing from 382 million MegaWatt hours

(MWh) in 2008 to 742 million MWh in 2018, contributing approximately 18% of total power generated in the United States in 2018. 13% (96 million MWh) of the total renewable power is solar from both small-scale and utility-scale installations. Small-scale installations typically include solar panels attached to buildings or other structures. Utility-scale installations are designed to supplement the power from

Figure 1. Illustration of a single-axis tracker system and its components.

the electricity grid; therefore, they consist of several rows of Photovoltaic (PV) modules. With a forecasted increase in the number of utility-scale installations and limited standardized design guidance for structural engineers to draw from, this article reviews the load criteria and the lessons learned from failures observed with such installations in the past decade.

The growth in demand for rooftop solar installations resulted in the development of the Structural Engineers Association of California’s (SEAOC) PV2-2012, Wind Design for Low-profile Solar Photovoltaic Arrays on Flat Roofs, followed by the inclusion of loading provisions in the American Society of Civil Engineers’ ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, in 2016. SEAOC subsequently updated its report in 2017, referencing the provisions of the published ASCE 7-16 standard. These resources provide guidance on the loading and behavior of small-scale rooftop solar units; however, limited guidance is available for the design of utility-scale PV solar structures.

the tracker systems (fixed, SATs, or DATs) are, at any given point, at an angle to the ground and, therefore, subjected to forces from the oncoming wind. Hence, the similarities to these equivalent structures seemed reasonable on the surface. Unfortunately, this practice resulted in wind-induced failures of tracker systems exposed to winds that were substantially lower than ASCE 7-16 design wind speeds.

A better understanding of the assumptions used to develop the provisions associated with monoslope roofs is needed to explain why the behavior of SATs under wind loading is not the same as monoslope roofs. The various causes of wind-induced failures of solar trackers and the lessons learned from multiple investigations are discussed below.

Utility-Scale PV Solar

SATs Failures and Lessons Learned

Utility-scale PV solar installations consist of multiple rows, each housing several PV modules mounted on a structural supporting frame.

Depending on the nature of this support system, these installations are classified as either Fixed-mount, Single-axis tracking (SAT), or Dual-axis tracking (DAT) systems. Fixed-mount systems consist of a supporting frame that is static and fixed, usually at an angle to the horizontal. In such systems, the angle of the solar panels relative to the ground stays constant in operation. In contrast, SATs and DATs enable the modules to “track” the sun during the day for enhanced efficiency. This improvement in efficiency is achieved by gradually adjusting the inclination and orientation of the modules during the day to achieve optimum sun exposure. SATs enable tracking in the EastWest direction only, while DATs enable tracking in other directions.

Given the lack of guidance for the design of the tracker systems, design engineers noticed the similarities between these support structures and other structures covered in ASCE 7. Engineers applied the loading provisions of these other structures to the design of tracker systems, primarily for the wind load case. Most popular among these equivalent structures are monoslope free roofs. The panels on any of

SATs consist of a tube section (called a torque tube), typically oriented with its longitudinal axis in the North-South direction (Figure 1).

This torque tube supports regularly spaced lateral frames or purlins designed to accommodate the PV modules. During operation, the torque tube rotates about its longitudinal axis, positioning the PV modules to track the sun during the day. The rotation of the torque tube is either provided by a motor mounted on the tracker torque tube itself or through multiple rows connected to a lever arm. In both setups, the rotation of the trackers is controlled centrally (and associated with in-situ weather monitoring) to obtain uniform tracking of all rows within the PV field. Enabling rotation about the torque tube to track the sun unlocks the torsional degree of freedom not commonly considered. The interaction of this additional degree of freedom with the oncoming wind has led to unexpected failures.

STRUCTURE magazine

Aeroelastic Instability Aeroelasticity is the study of the interaction of aerodynamic, inertial, and elastic effects on a body or a system. Instability resulting from

this interaction is typically characterized as an aeroelastic instability. The elastic effects (for the aeroelastic instability) originate from the torsional flexibility of the tracker systems. Increasing the stiffness to avoid instability inadvertently results in increased member sections that are not fully utilized in the strength limit state. The solution to this instability is an optimization problem, balancing the need for structural stiffness Figure 2. Comparison of sample wind tunnel results with ASCE 7-16 coefficients for angle with wind less than 5°. with underutilized strength.

Aerodynamics increases the available solutions to this problem. To designed is available, structural engineers tasked with the design of reduce the effective area subjected to wind, and corresponding wind tracker systems should consult a wind engineer for appropriate loadloads, tracker systems are usually “stowed� flat (0-degrees or parallel ing to minimize liability risk. to the ground) at higher wind speeds while not in operation. One of the aerodynamic solutions to the instability is to stow the PV modules Connections and Bolts at larger angles for higher wind speeds. Several tracker manufacturers consulted with wind engineers to devise optimal solutions that work A typical solar field incorporates thousands of tracker elements and best with their respective systems to resolve the instability. associated framing, and ease of installation is an essential factor in Most of the commonly observed wind-induced failures of tracker the design of the modern tracker system. Hence, a typical tracker systems exhibited evidence of an aeroelastic instability. Post-failure system includes pretensioned bolts that can be easily installed onsite. investigations typically show evidence of rows either completely dis- A common observation in several failed (and some operating) solar placed or a strong spatial concentration of displaced or damaged panels plants is a loss in pretension and loosening of bolts. Sometimes, poor towards the rotationally free end of the torque tube. The aeroelastic installation during erection is the culprit for loose bolts. In-service phenomena observed with trackers ranged from the most common vibrations due to wind response, albeit small, can also lead to a loss torsional galloping to a rarer torsional divergence. The mechanics of in pretension and bolt loosening. Loose bolts allow for unintended these phenomena are intricate and were partly introduced previously relative movements, which often compromise the stiffness of the through the discussion of the failure of the Tacoma Narrows Bridge tracker structural system and lead to undesirable aeroelastic effects.

(Cheruka, 2018). Torsional modes of failure similar to the bridge, The designer can take some measures to better control the in-service albeit lower mode numbers, are observed during field monitoring of vibrations and minimize the risk of bolt loosening. An understandSATs. These effects are beyond the scope of the ASCE 7 standard, ing of the buffeting response (response to a sudden random impulse) and limited design guidance is available to resist them. of the trackers and the use of dampers successfully controlled these vibrations and limited bolt loosening in the field.

Another common observation with the pretensioned bolts on these Load Magnitude trackers is bolt slip. The use of galvanized bolts for corrosion protecIn addition to the aeroelastic instability effects, the magnitude of the tion leads to an increased susceptibility for slippage of pretensioned applied wind loading is considerably influenced by the aeroelastic bolts. Similar to loosening, bolt slip can lead to a host of issues rangeffects of the tracker systems. These effects are one of the primary ing from relative movement at the splices to a deviation from the reasons why applying loading provisions for equivalent structures design angle of stow. from ASCE 7 to the design of SATs is not appropriate. The dynamics of monoslope free roofs and freestanding signs, considered when Load Application drafting the provisions in ASCE 7 standard, are not the same as the dynamics of SATs. The ASCE 7-16 monoslope free roof provisions Tracker systems are long and relatively flexible compared to commonly assume that all edges of the monoslope roof are structurally supported. designed buildings, primarily in the torsional degree of freedom. This In contrast, PV module systems are supported by a central support flexibility produces large rotations under load application, typically at that allows rotation about a central axis. This fundamental difference the ends of the tracker torque tube. This rotation may occasionally be in support conditions and associated movement under wind loads so large that the effective angle of attack (angle of the panels relative leads to significant differences in the applied load magnitudes. Figure 2 to the wind) may vary. For example, if the load application (load compares the pressure coefficient, GCp, in ASCE 7-16 with some magnitude at 0 degrees) results in a 15-degree rotation at the end sample wind tunnel results. As evidenced, the present ASCE 7-16 of the tracker, then the magnitude of load applied must be adjusted provisions for monoslope free roofs do not adequately represent the to accommodate the deviation from 0-degrees. In short, a nonlinear observed loading on a tracker in a wind tunnel. load application due to relative movement along the length of the Presently, there is a proposal under development, based on consensus tracker should be considered in the design. The extreme case of this wind tunnel coefficients from several laboratories, to incorporate wind is the static instability of torsional divergence, where the system never load provisions for ground-mount solar assemblies into ASCE 7. converges to a stable position. Even if stable, the applied loads may vary However, until standardized design guidance for the system being considerably and should be accounted for. When loads are procured NOVEMBER 2020

Figure 3. Results of non-linearity on tracker loading.

the frequency of the system is reduced (or the speed increased), the pressure-based methods tend to under-predict the peak torque indicating that stiffness is coupled with the observed loading. Hence, to accurately assess the wind load acting on the system, the buffeting response of the system must be evaluated using either numerical buffeting response analysis or aeroelastic model testing.

From a structural modeling standpoint, the conservative approach of modeling the support as a pin connection will not work because the system would be unstable (as the other end is torsionally free). In contrast, modeling the support as a fixed end may be unconservative and underestimates applied wind loads. It is thus advised to understand the motor system that is used to control the tracking. The connection between the motor and the torque tube should be studied to develop a reasonable estimate of the torsional stiffness at the support. It is also essential that the wind consultant is aware of the support stiffness conditions since there is a potential to affect the measured wind pressures. In-situ dynamic testing of mock-ups or test setups for natural frequencies have been performed by some system manufacturers to accomplish this.

from wind tunnel testing, the wind engineer must be consulted about this aspect of nonlinear load application to determine if this behavior is already accounted for in the provided loads.

Figure 3 shows the increase in the applied load magnitude due to nonlinear effects. The figure is based on an HSS 4×4×1⁄8 torque tube under a uniform torsion of 1 kip-in/ft along the length of the torque tube. In this case, the cumulative nonlinear torque at the fixed end of the tube and the rotation at the free end of the torque tube is 1.8 times the values calculated using linear analysis.

Summary Another factor often missed in load application is stow tolerance. The trackThe rapid growth in the demand for utilityers are stowed while not in operation or scale solar power led to an increase in the under stow conditions (typically high construction of the trackers across the U.S. wind speed events) to ensure safety. For However, design standards have not kept up example, a tracker stowed flat (or 0 degrees with this surge in demand, leading to multiple to the ground) at high wind speeds has a failures. From lessons learned through invesFigure 4. Schematic of aerodynamic loading as a less effective area and thus observes lower tigation of failures, the designer can mitigate function of reduced frequency. wind loads. However, this stow position is the risk of failure by: not always exact, and some systems associate a tolerance as high as • Understanding the aerodynamics and/or consulting a wind ±7.5 to 10 degrees. The structural engineer should verify the behavior engineer for potential instability, estimate of load magnitude, (and safety) under stow conditions, including the stated tolerance. and load application procedure; • Understanding the tracker system by learning the operational stow tolerance and support fixity; Support Fixity • Considering in-service wind-induced vibration of the tracker As discussed previously, SATs primarily act as cantilevers in the torand its influence on various components; and, sional degree of freedom. However, it should be noted that the “fixed” • Designing connections to ensure limited stiffness discontinuiend of the cantilever is not always fixed. Often, they are connected to ties by minimizing bolt slip or loosening.■ a motor or a lever arm that is used to control rotations during the day.

The torsional restraint of these end connections is not sufficient to be References are included in the PDF version deemed as a “fixed” support. The flexibility of the sole torsional support of the article at STRUCTUREmag.org. leads to challenges in the design. Deviation from fixity at the torsional support reduces the natural frequency of the system. Figure 4 shows the peak torque as a function of reduced frequency estimated using Sumanth Cheruku is a Project Engineer with Thornton Tomasetti in their different methods. Reduced frequency is a non-dimensional number Austin office. He is the Secretary of Chapter 29 of ASCE 7 for the 2022 to combine the effects of the stiffness of the system (frequency, f ), code cycle and has investigated failures and peer-reviewed designs for across wind dimension (L), and the wind speed (V). Since peak wind utility-scale PV solar trackers from various manufacturers. load magnitudes increase as natural frequency decreases, assuming Matthew T.L. Browne is a Principal and Technical Director in RWDI’s fixity is not conservative.

Wind Loading.

Pressure-based methods use rigid models in the wind tunnel, and dynamic properties of the structure are mathematically modeled. As STRUCTURE magazine

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NCSEA NCSEA News

National Council of Structural Engineers Associations

Congratulations to the 2020 NCSEA Special Awards Honorees The Special Awards honor individuals who have provided outstanding service and commitment to the association and to the structural engineering field. These awards are bestowed on individuals who exemplify the volunteerism that has brought NCSEA to where it stands today. Visit www.ncsea.com to learn more about the Special Awards and about this year’s recipients.

NCSEA Service Award

Robert Cornforth Award

This award is presented to an individual who has worked for the betterment of NCSEA to a degree that is beyond the norm of volunteerism. It is given to someone who has made a clear and indisputable contribution to the organization and the profession.

This award is presented to an individual for exceptional dedication and exemplary service to an NCSEA Member organization as well as to the structural engineering profession.

Brian Dekker, P.E., S.E., is president of Sound Structures. He has served in many leadership positions within NCSEA, including chair of the Advocacy Committee and positions as Director, Treasurer, Vice President, President, and Past President of the Board of Directors. During his presidency he was instrumental in many new initiatives, including the Member Organization Grant Program. Brian took on a new volunteer role this year as the President of NCSEA Media, leading the business operations of STRUCTURE magazine. Brian’s past and current roles at NCSEA have benefited the association, our member organizations, and the entire field.

Jerry Maly, P.E., is a civil/structural engineer and a Principal in the Denver, Colorado office of Wiss, Janney, Elstner Associates, Inc., where his practice is focused on the rehabilitation of existing buildings and the investigation and repair of structural damage, deterioration, and failures. Jerry served SEAC as an Officer from 1996-1999 and as Chair of the Membership Committee for over twenty years, welcoming more than 500 new members. For over fifteen years, he has served as chair of the SEAC Denver Building Department Liaison Committee, representing the structural engineering community in the Denver Code Adoption process.

Susan M. Frey NCSEA Educator Award

James Delahay Award

This award, established to honor the memory of Sue Frey, one of NCSEA’s finest educators, is presented to an individual who has a genuine interest in, and extraordinary talent for, effective instruction for practicing structural engineers.

This award is presented at the recommendation of NCSEA’s Code Advisory Committee to recognize outstanding individual contributions towards the development of building codes and standards. It is given in the spirit of its namesake, a person who made a long and lasting contribution to the code development process.

Duane K. Miller, ScD, P.E., is a recognized authority on the design of welded connections. He has received the American Institute of Steel Construction’s T. R. Higgins Lectureship Award, the Lifetime Achievement Award, the Robert P. Stupp Award for Leadership Excellence, and was the fi rst recipient of the Steel Conference Speaker Award. He has authored and coauthored many texts, including the AISC Design Guide on Welding and the Mark’s Handbook of Engineering, 10th Edition.

Kevin Moore, P.E., S.E., SECB, is a Senior Principal and Structural Division Head of Simpson Gumpertz & Heger Inc. He has been active in NCSEA since 2003, and is the current Chair of the NCSEA Resilience Committee and Past Chair of the NCSEA Seismic Subcommittee to the Code Advisory Committee. He is the chair of the Structural Standards Committee of SEAOC and is the Vice President of the SEAONC Board. Kevin represents NCSEA with the Building Seismic Safety Council and serves on a number of standards development organizations.

STRUCTURE magazine

News from the National Council of Structural Engineers Associations

2020 MO Public Outreach Challenge

NCSEA’s External Communications Committee is expanding last year’s MO Public Outreach Challenge to allow members, delegates, and SEA leadership to submit their Member Organization’s most effective program for achieving NCSEA’s goal that: Practicing structural engineers are recognized by clients, media, policymakers, educators, students, and the public for the value of their contributions to society.

The purpose of this competition is to give SEAs a platform to be recognized by the SE profession for their work and dedication to the field. What do you consider your SEA’s biggest Public Outreach accomplishment in the past year (November 2019 through December 2020)? Each SEA may submit up to one entry for each of the following categories: public outreach; political advocacy (influencing public policy, providing educational opportunities supporting political actions, etc.); students and educators (forming partnerships with schools, offering scholarships or mentoring programs, etc.); or other outreach programs related to current NCSEA initiatives, including collaboration with allied organizations and diversity, equity, and inclusion efforts.

Submissions are due December 15, 2020. Visit www.ncsea.com for more information.

Maria Mohammed accepting the first place award for the 2019 Challenge on behalf of the Structural Engineers Association of California

2020 NCSEA SEA Grant Program Open for Applications

The NCSEA Grant Program was developed in 2015 to award SEAs funding for projects that grow and promote their SEA and the structural engineering field in accordance with the NCSEA Mission Statement. This program is now funded by the NCSEA Foundation, which was established early this year to further support the non-profit activities of NCSEA and its Member Organizations. The goal of the Foundation and the Grant Program are to advance the structural engineering profession through technical development, education, and outreach.

The Recipients of the 2019 Grant Program were: • Structural Engineers Association of Central California to enhance their new Structural Engineering, Engagement, and Equity (SE3) Committee • Structural Engineers Association of San Diego to support the EERI San Diego-Tijuana Regional Earthquake Scenario Study and a Special Wind Region Study • Structural Engineers Association of Illinois to host a Young Professionals Workshop • Structural Engineers Association of Kansas/Missouri to launch an SE3 Committee Panel Discussion and Networking Event and to assist with STEM classes for local elementary school students

• Structural Engineers Association of New York for a screening of the documentary Leaning Out with panel • Structural Engineers Association of Ohio for a Young Members’ Track at SEAoO’s Annual Conference • Oklahoma Structural Engineers Association to assist OSEA’s efforts in the Engineering Fair E-week 2020 Bridge Competition • Structural Engineers Association of Texas to support a local SE3 Speed Mentoring event • Structural Engineers Association of Washington to assist with a Joint Special Regions Wind Study

• Structural Engineers Association of Massachusetts to launch an SE3 Committee Interactive Seminar Series Applications are due December 1, 2020. Application requests must be reviewed and approved by the Member Organization before being submitted to NCSEA for consideration. Learn more about the Grant Program by visiting www.ncsea.com.

NCSEA Webinars

Register by visiting www.ncsea.com.

November 12, 2020

Warning Flags For Structural Engineers – Hidden Contractual Risk

December 3, 2020

Structural Planning of Gravity and Lateral Load Resisting Systems

Rob Hughes, Esp

Sudarshan Krishnan

November 19, 2020

December 10, 2020

Fundamentals of Structural Planning Sudarshan Krishnan

Retrofitting of Existing Buildings with Steel Joists Bruce Brothersen, P.E., S.E., and Walter Worthley, Jr., P.E.

Courses award 1.5 hours of Diamond Review-approved continuing education after the completion of a quiz.

NOVEMBER 2020

SEI Update Learning / Networking

SEI Virtual Events

www.asce.org/SEI/virtual-events • Join the discussion with leaders on Performance-Based Design #SEILive on YouTube – Wednesday, December 2, 12:30 pm US ET • Career Path Series: Insights with Glenn Bell and SE Industry Leaders SEI/ASCE Members have free access to sessions and resources online.

NEW in the ASCE Bookstore and Library Guide to the Tsunami Design Provisions of ASCE 7-16

By Ian N. Robertson, Ph.D., S.E. Available at www.asce.org

Significant Papers from SEI Journals Check out editor-selected influential papers that have been instrumental in moving civil engineering forward or have changed the practice of structural engineering from the Journal of Bridge Engineering, the Journal of Structural Engineering, and the Practice Periodical on Structural Design and Construction. https://ascelibrary.org/influentialseipapers

Errata STRUCTURE magazine

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata.

If you would like to submit errata, contact Kelly Dooley at kdooley@asce.org.

News of the Structural Engineering Institute of ASCE Advancing the Profession

New SEI Committee Charges

At the September 28 SEI Board of Governors meeting, charges were approved for these two committees in line with SEI Strategic vision initiatives: SEI Diversity, Equity, and Inclusion (DEI) Committee The DEI Committee shall support SEI ASCE goals, values, and principles by: Seeking to measurably advance the profession of Structural Engineers by promoting DEI to Structural Engineers and SEI through the: a. Education; b. Practice; and, c. Business of Structural Engineers and the profession, where advancement of the profession is achieved

by Structural Engineers including DEI in their practice of being the “stewards of the built environment, [in which] structural engineers are widely recognized as making key contributions to the advancement of society on a national and global scale.” (CASE, NCSEA, SEI joint statement.).

• Introduce DEI as a value of Structural Engineers and the profession of structural engineering.

• Collaborate with other organizations (e.g., CASE, NCSEA, other Society Institutes, educational and others) to create initiatives based on DEI shared values relevant to Structural Engineers, and work to disseminate those initiatives through SEI’s community.

Staff contact Justin Blan at jblan@asce.org.

SEI Committee on Resilience To direct, oversee, and coordinate SEI’s efforts to enhance resilience for buildings, bridges, and other structures by preparing recommendations for the SEI Board of Governors and SEI Divisions, Committees, and Members.

Staff contact Jennifer Goupil at jgoupil@asce.org.

Call for Members

New Loading Standard for Structures Supporting Overhead Power Lines & Telecommunications Infrastructure

Development of a new standard ASCE/SEI XX, Minimum Design Loads for Structures Supporting Overhead Power Lines & Wired Telecommunications Infrastructure, will begin in 2021. The committee seeks new members thru the end of 2020 to begin work on the new edition of the standard.

Michael Miller, P.E., M.ASCE, Vice President of Engineering and Business Development at SAE Towers, Ltd., will chair the cycle. Practicing engineers, researchers, building officials, contractors, and construction product representatives are needed and welcome. Apply to join the committee by December 31, 2020, via https://bit.ly/30OqQRQ. Select SEI from the Institute drop-down and then “Minimum Design Loads for Structures Supporting Overhead Power Lines & Wired Telecommunications Infrastructure.” Carefully indicate the Membership Category for which you are applying. Associate members can be accepted until balloting begins. Eligible regulatory members can qualify for travel reimbursement per ASCE Travel Policy when applicable. Contact Jennifer Goupil at jgoupil@asce.org with questions.

Design of Steel Transmission Pole Structures Standard (ASCE 48)

The revision cycle for the Design of Steel Transmission Pole Structures (ASCE 48) will begin in 2021. The committee seeks new members to begin work on the next edition of the standard. Nick Grossenbach, P.E., A.M.ASCE, Manager of Engineering at American Transmission Co., will chair the next cycle. Practicing engineers, researchers, building officials, contractors, and construction product representatives are all needed and welcome. Apply to join the committee by December 31, 2020, via https://bit.ly/3jH9SfG. Select “SEI” from the Institute drop-down and then “Steel Transmission Pole Structure (ASCE/SEI 48).” Carefully indicate the Membership Category for which you are applying. Associate members can be accepted until balloting begins. Eligible regulatory members can qualify for travel reimbursement per ASCE Travel Policy when that occurs. Contact Jennifer Goupil at jgoupil@asce.org with questions.

SEI Online

ASCE Failure to Act – Electricity Report

Check out the report and remarks by Otto Lynch, P.E., F.SEI, F.ASCE in the release event webinar at www.asce.org/electricity_report

CROSS-US

View the latest newsletter and reports www.cross-us.org.

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle N O V E M B E R 2 02 0

CASE in Point Did you know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several tools available for firms to use for recruiting and retaining employees.

962 962-B Tool 2-2 Tool 2-3 Tool 2-6 Tool 3-2 Tool 3-5 Tool 4-3 Tool 5-1 Tool 5-2

National Practice Guidelines for the Structural Engineer of Record National Practice Guidelines for Specialty Structural Engineers Interview Guide and Template Employee Evaluation Templates Structural Engineering Job Descriptions Staffing and Revenue Projection Staffing Schedule Suite Sample Correspondence Guidelines A Guide to the Practice of Structural Engineering Milestone Checklist for Young Engineers

NEW CASE Publication Released!

CASE Tool 2-6: Structural Engineer Job Descriptions

When targeted to people outside the firm, well-written job descriptions entice the most qualified people to apply with your firm. To get the most qualified candidates, list quantitative and qualitative requirements such as experience, education, and desired personality traits. These types of qualifications help to eliminate undesirable candidates.

When targeted to people inside the firm, job descriptions can be utilized as a powerful management tool. The details contained in well-written job descriptions form the basis for developing a clear understanding between the employee and the manager of the employee’s expectations.

Managers can also use the terms in the job description to determine how the employees performed when conducting performance appraisals. The criteria used for performance evaluations ideally would match the expectations listed in the employee’s job description. The job description for the position above the employee’s current position can explain what is required for that person to earn a promotion.

The job descriptions in Tool 2-6 are intended to be used as a template for creating job descriptions for your firm. Word files are provided, with detailed descriptions and a matrix with abbreviated descriptions when comparing engineering levels.

Popular CASE Guideline Updated!

CASE 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction Documents CASE has released a comprehensive update to its popular: Guideline Addressing Coordination and Completeness of Structural Construction Documents. The guideline will assist the structural engineer of record (SER) and everyone involved with building design and construction in improving the process by which the owner is provided with a successfully completed project. Their intent is to help the practicing structural engineer understand the importance of preparing coordinated and complete construction documents and to provide guidance and direction toward achieving that goal.

This guideline focuses on the degree of completeness required in the structural construction documents (“Documents”) to achieve a “successfully completed project” and on the communication and coordination needed to reach that goal. They do not attempt to encompass the details of engineering design; instead, they provide a framework for the SER to develop a quality management process.

Currently, the coordination and completeness of Documents vary substantially within the structural engineering profession and among the various professional disciplines comprising the design team. The SER’s goal should be meeting both the owner’s and the contractor’s needs by producing a complete and coordinated set of Documents. Owners and contractors generally understand that, for some Documents, changes will occur because they realize that no set of Documents is perfect. The SER must focus on completeness, coordination, constructability, and reducing errors to minimize potential changes.

An overall comprehensive update was done to the document to keep with best business practices and current industry standards.

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

STRUCTURE magazine

News of the Coalition of American Structural Engineers And the Scholarship Winner Is….

The CASE scholarship, administered by the ACEC College of Fellows, is awarded every year to a deserving student seeking a master’s degree in an ABET-accredited engineering program.

Since 2010, the CASE Scholarship program has given over $34,000 to engineering students to help pave their way to a bright future in structural engineering. CASE strives to attract the best and brightest to the structural engineering profession, and educational support is the best way to ensure our profession’s future.

The 2020 winner, Amanda Kalab, will graduate May 2021 with a master’s degree in Civil Engineering, with an emphasis in Structural Engineering, from Washington State University.

ACEC Coalition Virtual Education Series

The next education session will take place on Thursday, December 3rd, from 1:30 pm to 3:00 pm. Hear from legal expert Kent Holland of ConstructionRisk, LLC, who will provide a year-end review of Construction Law. To register for this session, go to www.acec.org/coalitions/upcoming-coalition-events. The series will continue in January and will cover the insurance industry. Stay tuned for more information in next month’s edition.

NEW – Strategies for Developing a Respectful, Diverse, and Inclusive Workplace Culture Employers are under significant scrutiny for the environment that exists in their workplaces.

In recent years, #MeToo, systemic racism, gender inequality, generational differences, and negative behaviors in the workplace have posed enormous challenges for managers that, if ignored, can result in lack of engagement, attrition, and even lawsuits.

This course is designed to help those in management positions learn how to address these challenges by developing a culture of respect and inclusion. When respect thrives in the workplace, so does an engaged staff commited to excellence.

This four-module course combines the scheduling ease of video learning and the immediacy and intensity of a live classroom.

• Access recorded lectures anytime via computer, tablet, or smartphone • Attend weekly live discussions with an instructor via WebEx Meeting • Work together on small group assignments Participants will earn a minimum of 8 PDHs too!

The program begins on January 25, 2021. Only 40 seats – register today at https://education.acec.org/diweb/catalog/item?id=6096116.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

NOVEMBER 2020

SOFTWARE updates ASDIP Structural Software

ENERCALC, Inc.

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Digital Canal Corp.

Phone: 800-449-5033 Email: info@digitalcanal.com Web: www.digitalcanalstructural.com Product: Cold Formed Steel Design Description: Please visit the website for New Cold Formed Steel Design software information. You receive exceptional value with a one-project return on your investment. You also own your licenses. We do not force ongoing payments forever. Our 11,000 clients provide the best testimonial we can offer.

DuraFuse Frames Phone: 801-727-4060 Email: contact@durafuseframes.com Web: www.durafuseframes.com Product: DF360 Description: Unique seismic resilient systems preventing beam and column damage while also providing repairability. DuraFuse Frames offers the ideal moment-frame and dual-frame solutions for all building types in all Seismic Design Categories and provides the most versatile SMF/IMF system on the market in addition to resiliency.

Listings are provided as a courtesy, STRUCTURE is not responsible for errors.

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STRUCTURE magazine

IES, Inc. Phone: 406-586-8988 Email: info@iesweb.com Web: www.iesweb.com Product: IES Building Suite Description: You need practical, affordable tools that really help you solve analysis and design problems efficiently. Fortunately, with IES, Inc., you will get high quality reports from easy-to-use software. Find out for yourself why thousands of engineers rely on VisualAnalysis, VisualFoundation, ShapeBuilder, QuickRWall and our other excellent products.

Nanometrics Phone: 855-792-6776 Email: sales_mkt@nanometrics.ca Web: www.nanometrics.ca Product: Vantage Structural Health Monitoring Description: An integrated solution for monitoring the structural integrity of critical infrastructure and facilities, the Vantage Structural Health Monitoring portal provides immediate notification of significant events along with automatically compiled incident reports for rapid assessment of any impact to a facility.

RISA Phone: 949-951-5815 Email: benf@risa.com Web: www.risa.com Product: RISAFoundation Description: The latest release of RISAFoundation (Version 13) includes full integration with the RISA Building System and adds new features such as expanded load categories, the ability to exclude results, code updates to ACI, and enhanced seismicity. Visit risa.com/new-features/risafoundation to learn about all the new features.

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STRUCTURE NOVEMBER 2020

Bonus Content

business PRACTICES Market Trends – Succeeding Into 2022

By Kacey Clagett, LEED AP BD+C

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he economic downturn caused by the Covid-19 pandemic has affected design and construction firms around the United States, whether by stopping or slowing projects, triggering overnight switches from normal business operations to working from home, or its tolls on personal, physical, and emotional health. As the pandemic wears on, businesses are finding ways to overcome these setbacks. Yet, most experts expect that real recovery may not take place until 2022.

This article points out strategies that can help your firm be resilient over the near term and positioned for continued financial success.

Where We Are Since the Covid-19 pandemic took hold in March of this year, the national economy fell 31.7% (on an annualized basis) in the second quarter of 2020, the worst drop ever recorded by the U.S. Department of Commerce’s Bureau of Economic Analysis.

This was preceded by a 5% contraction in the national gross domestic product in the first quarter of 2020. Many engineering and architecture companies have experienced hardship.

The American Institute of Architects’ AIA Billings Index (ABI), considered an important economic indicator for all nonresidential construction activity, including multi-family, for the next nine to 12 months, plummeted from an overall score of 53.4 in February 2020 to 29.5 in April 2020. The AIA Billings Index looks at a month’s performance against the previous month, and a score of 50 indicates no change. Anything below 50 indicates a decreased outlook from the month before; over 50 shows growing demand. The August 2020 ABI, released in late September 2020, had an overall score of 40.0. The ABI has stood at 40.0 since June 2020.

Before implementing shelter-in-place measures, the majority of regions and market sectors were relatively healthy, with the strongest outlooks in the Southeast and the West.

Most market sectors were in reasonably good shape, with some softness in higher education from the “baby bust,” the drop off in births during the 1990s, and its outsized inflation relative to the overall economy. Industrial and multi-family were particularly strong. In July 2020, Appleseed Strategy released results from its survey of 31 design and construction STRUCTURE magazine

companies with offices in 12 states around the country. Compared to the months immediately before the shelter-in-place measures began, two-thirds of the respondents had experienced some drop off in their normal billings and projections as of late June 2020.

While 30% of respondents had decreases in billings of 20% or less, 10% of the respondents had their billings decrease by over 60%. August 2020 American Institute of Architects ABI Index.

Firms experiencing greater drop-offs were more likely to be located There are pockets of more robust markets. in the Northeast. All survey respondents were The Federal Government can choose to selfconcerned about their pipeline projects, with fund its projects and assist state and local 67% of the companies facing decreasing governments. Generally, Federal agencies have projections, and an overall 42% projected continued work or are “kicking the tires” with decreases of 20% or more. feasibility studies. Many are planning for future The respondents expressed uncertainty in work, particularly through IDIQs (Indefinite their pipelines just one or two months out, Delivery, Indefinite Quantity solicitations, also in contrast to the more traditional reliability known as as-needed services contracts), which of these numbers for three to six months into continue to be advertised. Grocery stores, drugthe future. stores, warehouses, and distribution centers are busy. Commercial and institutional real estate owners are looking at repurposing their Outlook – Uneven Terrain holdings, although this work is often limited Geographic regions and market sectors to feasibility and pricing studies. New and continue to be unevenly affected by the pan- renovated housing is in demand, often outdemic. The Southeast and West show some side of metropolitan centers. Owners across improvements, primarily driven by the con- all markets are trying to use this time of tinued long-term population shift to those decreased occupancy for needed repairs like regions. Much of the Midwest is not densely deferred maintenance and seismic upgrades, populated, which has reduced the immedi- with the hope that construction prices will ate effects of the pandemic. The Northeast, fall as the pandemic wears on. However, a corwhose economic indicators lagged behind rection in construction pricing is not likely to the West and Southeast before the spread take place until 2021, because most builders of Covid-19, was hardest hit in the early have contracts in place for materials and with months of the pandemic and, compared to subcontractors. Any lowering of construction other regions, is more built out. Commercial costs would happen once the subcontractor owners across the country are reassessing and vendor pipelines drop. In regions like the their real estate plans, especially in tourism, San Francisco Bay Area, many builders have hospitality, retail, and commercial office. solid pipelines into 2022.

Many government agencies and institutions are also in flux, whether from reduced fundWhat You Can Do ing and donations to capital campaigns or by the closure of in-person education and fears • Keep in touch with your current, of exposure at healthcare facilities. past, and prospective clients. This is

fundamental at any time, but even more critical now. Show interest in what they are experiencing and find ways to offer help. Consider conducting surveys or holding roundtables to provide them with information they can use.

• At the same time, take an objective look at your client list, and evaluate your profitability by client, sector, service offering, and similar metrics. Pay special attention to where you have greater than 20% of your revenue from a single client, market sector, or service. These are areas where companies are in most danger of experiencing hardship if something unexpected happens. Diversify as much as you can.

• Reassess your business plan. A downturn is a great period to test out new models.

Necessity is the mother of invention.

Consider new service offerings and pricing structures. It may be time to be even bolder, through strategic hires, alliances, and acquisitions.

• Watch your competitors. They are watching you. There is an old adage in business development: your best client is your rival’s best prospect. Beyond the need to keep on your toes, your competitors may be launching new business models or pricing structures. Talented staff will also be coming on the market; keeping an eye on your competitors may give you the chance to get new resources in areas that will make you more economically resilient.

• Business development accountability (and training) for everyone! A strong business nurtures its whole staff to participate in business development on some level, whether by acting as a company ambassador or by keeping an eye out for opportunities to cross- or up-sell services.

Remember that your key contacts may move on to new companies or retire, and your next client may be more receptive to others on your team.

Recommendations • Take advantage of existing federal, state, and local government assistance programs, such as grants and low-interest loans. Watch for new programs and changes to those already in place. Because 2020 is an election year, more government assistance will be a top priority, whether through grants, loans, or tax relief. Existing programs have already been modified to facilitate their use, and further changes are very possible.

• Like it or not, any recession opens the profession up to disruptive influences. While

brief lulls in work may be time to address nonbillable projects that have not received attention, downturns merit a look at where the market is headed and, consequently, strategic adjustments. Recessions are often when new businesses are launched, and these new competitors are likely to offer more aggressive risk/reward propositions. This is the time to look objectively at your business and consider refocusing or retraining staff, as well as making strategic hires, alliances, acquisitions, or mergers.

• Your clients are also evaluating what they can do to keep themselves financially successful. They are likely to be more open Appleseed Strategy Survey – Pipeline Predictions. to new ideas, especially when looking to Conclusions prove their worth to their own bosses.

For instance, while many well-known Whether you are experiencing little-to-no retail brands and shopping center owners change in your business, or are in hardship, have been devastated by the pandemic, use this period to evaluate how you can make Amazon is buying former retail spaces to your company more resilient and set for retrofit for local distribution. Across all future success. Research is essential. Look at regions and markets, owners are evaluatwhat has made you successful, or not, and ing how to make the best use of what how likely the future economy will support they have as they respond to economic your business model. Do the math. Talk to disruptions. Structural engineers may people – clients, prospects, allies, competifind it advantageous to contact owners tors, and especially your staff. Your financial directly about offering targeted consulta- success depends on an open mind and a tion bypassing the more expensive, full clear-eyed assessment of both your team approach. company’s health and a range of • While most experts agree that people possible future market conditions.■ have short memories and that many modifications wrought by the pandemic The online version of this article will fade away once we are past its probcontains references. Please visit lems, it is also likely to spur longer-term www.STRUCTUREmag.org. physical changes to building stock. An obvious one many companies face: how Kacey Clagett is a Principal and Founder much real estate do we need now that we of Appleseed Strategy. are used to working from home at least (kacey.clagett@appleseedstrategy.com) part of the time? In Appleseed Strategy’s Summer 2020 survey, 73% of respondents were contemplating reducing their office space and seeking rent reductions This article is based on findings from Appleseed and forbearance. Anecdotal evidence Strategy’s Summer 2020 financial and points to many companies maintaining economic outlook survey. It has been updated work from home policies through the with new economic data from the AIA Billings end of 2020 and beyond.

Index and other sources.

NOVEMBER 2020 BONUS CONTENT

Response to October 2020 STRUCTURE Article

By Jason B. Lloyd, Ph.D., P.E., Robert J. Connor, Ph.D., P.E., and Karl H. Frank, Ph.D., P.E.

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he October 2020 STRUCTURE article, Coating Preparations Reduce the Strength of Bridges, by Robert A. Leishear, Ph.D., P.E., PMP, presents information and opinions on potential problems with the fatigue resistance of steel

bridges prepared for coatings using grit blast cleaning methods. Some of the information in this article is misleading with unsubstantiated claims regarding the safety of existing and future steel bridges. These topics are addressed below.

Blast cleaning has been used in the coating process of steel bridges for decades. Shot and grit blasting techniques are approved cleaning methods used in fabrication shops, as well as field painting for new and existing bridges. The most common media used is a shot/grit mixture. The blast cleaning processes are regulated by state department of transportation specifications for bridge design or rehabilitation projects. These generally are consistent with the AASHTO LRFD Bridge Construction Specifications where it states in article 13.2.3.1 that blast cleaning “shall leave all surfaces with a dense and uniform anchor pattern of not less than 1 mil or more than 3 mils, as measured with an approved surface profile comparator” (AASHTO, 2017). The methods for removal of foreign material for surface preparation for liquid coatings generally also conforms to either the SSPC-SP 6 or SSPC-SP 10 preparation specifications with additional guidance provided by AASHTO/NSBA Steel Bridge Collaboration S8.1 (2014). Surface preparations for thermal spray coatings are performed in accordance with SSPC-CS 23.00/AWS C2.23/NASCE No.12, as well as additional Example large-scale fatigue test of steel bridge girders (Hebdon et al., 2017). guidance provided by AASHTO/NSBA Steel Bridge Collaboration S8.2 (2017), specifying a surface roughness between caused by the grit blasting relative to the polished surface is expected 2.5 and 5 mils. and an important consideration for machined components.

The opinions in the October 2020 article are based on the misapplicaThe October 2020 article stated that grit blasting “significantly tion of the work by Padilla, Velasquez, Berrios, and Puchi Cabrera of degrades the strength of steel bridges, endangering safe design.” This the University of Venezuela, which was published in 2002. The cited statement is based upon the reduction in the rotating beam specimens research is thorough and adept with an important field of application relative to a mirror-like surface observed by Padilla et al. However, the listed as dynamic components of helicopters. We do not take issue mirror-like surface commonly used in rotating beams tests is vastly with the research; however, we fully disagree with applying those different than the as-fabricated and as-rolled surface conditions of steel results to steel bridge fatigue life design and safety. used in highway and railway bridges. The fatigue design requirements The research by Padilla et al. (2002) included the comparison of in the AASHTO specifications are based upon full-scale girder tests fatigue life of rotating-beam specimens having three different surface with as-received mill scale surfaces (see Figure), as well as bolted conconditions; mechanically polished (described as “mirror-like”), grit nection tests with blasted and blasted-then-coated surfaces (Fisher et blasted, and grit blasted with hard facing thermal spray coating. The al., 1983; Fisher et al., 1974; Fisher et al., 1970; Brown et al., 2007; specimens were made from SAE 4140 steel (which is not a structural Frank and Yura, 1981). The research is conclusive; fatigue resistance steel used in bridges) with a measured yield strength reported as of all steel bridges is governed by welded or bolted connection details, approximately 127 ksi. The specimens were tested in a rotating beam not by minor surface conditions. This is particularly true at the low apparatus and were subjected to reversed bending at very high stress effective fatigue stress ranges experienced by in-service steel bridges, levels. This type of fatigue testing is sensitive to surface condition which, based on extensive field testing experience of the authors, is effects and yield strength. Thus, it would be sensible for a researcher typically only about 4 to 8% of the steel yield strength. Furthermore, to choose these relatively quick and affordable tests when wanting to the fatigue life of the rotating beam tests, performed by Padilla et al., observe the influence of different surface conditions on fatigue life for greatly exceeded the fatigue life of steel bridge welded connection a particular base material. The rotating beam tests were performed at details that are used throughout the United States. high stress ranges, including approximately 69, 74, 79, and 84 ksi (54, Extensive fatigue studies of bolted connections with blasted and 58, 62, and 66% of the yield strength, respectively). The elevated stress blasted-then-painted surfaces have been performed (Brown et al., ranges accelerate the fatigue testing and help amplify the influence 2007; Frank and Yura, 1981). These studies showed that the coated of minor surface condition parameters. The reduction in fatigue life specimens had a slightly higher fatigue resistance due to the reduction STRUCTURE magazine

in fretting caused by slippage of the connection. The uncoated blasted surface fatigue life equaled or exceeded the Category B fatigue design strength for bolted connections. These large-sized bolted connections, which included both weathering and non-weathering steel and realistic surface preparations, confirmed the adequacy of the AASHTO specifications.

It must be kept in mind that the current AASHTO fatigue design specifications are derived from experimental data representing the 95 percent confidence limit for an approximate 97.5 percent survival for each detail type. Extensive fatigue data was accumulated over many years of testing to develop the AASHTO categories. The fatigue design curves statistically correspond, therefore, to the shortest lives experimentally observed for each category, which, of course, would have been governed by the most severe discontinuity. What resulted are AASHTO fatigue design curves representing the detail with the most severe discontinuity and predicting, with high statistical confidence, that it will survive the desired service life. This also means that a substantial majority of details in a given category will have longer fatigue lives than predicted by a design curve.

There is an extensive experimental database that was used to develop the AASHTO fatigue design provisions, which are based upon

large-scale test specimens having surface conditions, constraints, residual stresses, random flaw distributions, and welding procedures used for actual bridges. An extrapolation of rotating-beam fatigue test data for surface roughening to the fatigue behavior of some industries may be acceptable, but it is inappropriate for fabricated steel bridges.

Likewise, a claim that bridge designs are “in jeopardy” due to fatigue is egregious. The claimed reduction in fatigue strength has not been found in large-scale fatigue tests of bridge components nor in the observed excellent in-service fatigue performance of steel bridges over the past 45 years.■ Jason B. Lloyd is a Bridge Steel Specialist with the National Steel Bridge Alliance.

Robert J. Connor is the Jack and Kay Hockema Professor in Civil Engineering and the Director of the Center for Aging Infrastructure and S-BRITE Center at Purdue University.

Karl H. Frank is a Professor Emeritus at The University of Texas at Austin, the Chief Engineer at Hirschfeld Industries, Ret., and a Consultant.

References AASHTO (2017). LRFD Bridge Construction Specifications, 4th Edition. American Association of State Highway & Transportation Officials. Washington, DC. AASHTO/NSBA Steel Bridge Collaboration. (2017). S8.2 Specification for Application of Thermal Spray Coating Systems to Steel Bridges. Available online: https://store.transportation.org/Item/PublicationDetail?ID=3910 AASHTO/NSBA Steel Bridge Collaboration. (2014). S8.1 Guide Specification for Application of Coating Systems. Available online: https://store.transportation.org/item/publicationdetail/2355 Barsom, J. M. (1981). Fatigue Considerations for Steel Bridges. Fatigue Crack Growth Measurement and Data Analysis, ASTM STP 738, S. J. Hudak, Jr., and R. J. Bucci, Eds., American Society for Testing and Materials, pp. 300-318.

Brown, J. D., Lubitz, D. J., Cekov, Y. C., and Frank, K. H. (2007). Evaluation of Influence of Hole Making Upon the Performance of Structural Steel Plates and Connections. Report No. FHWA/TX-07/0-4624-1. University of Texas at Austin. Austin, TX. Fisher, J. W., Mertz, D. R, and Zhong, A. (1983). Steel Bridge Members under Constant Amplitude Fatigue Loading. NCHRP Report 267. Transportation Research Board. Washington, DC. Fisher, J. W., Albrecht, P.A., Yen, B. T., Klingerman, D. J., and McNamee, B. M. (1974). Fatigue Strength of Steel Beam with Welded Stiffeners and Attachments. NCHRP Report 147. Transportation Research Board. Washington, DC. Fisher, J. W., Frank, K. H., Hirt, M. A., and McNamee, B. M. (1970). Effect of Weldments on the Fatigue Strength of Steel Beams.

NCHRP Report 102. Transportation Research Board. Washington, DC. Frank, K.H., and Yura, J.A. (1981). An Experimental Study of Bolted Shear Connections, Report No. FHWA RD-81/148, University of Texas at Austin, Austin, TX. Hebdon, M. H, Bonachera Martin, F. J., Korkmas, C., and Connor, R. J. (2017). Load Redistribution and Remaining Fatigue Life of Steel Built-up Members Subjected to Flexure Following a Component Failure. Journal of Bridge Engineering, ASCE. DOI: 10.1061/(ASCE)BE.1943-5592.0001087.

Padilla, K., Velasquez, A., Berrios, J. A., and Puchi Cabrera, E. S. (2002). Fatigue Behavior of a 4140 Steel Coated with A NiMoAl Deposit Applied by HVOF Thermal Spray. Elsevier Press, Vol. 150, pp. 151-162. Amsterdam, Netherlands.

NOVEMBER 2020 BONUS CONTENT